Colour Theory

Colour theory is a set of basic rules for mixing colour to achieve a desired result.

But What is Colour?

If light bounces off an object and there is no human there to see it, does it have colour? This is not a trick question, but one that makes you think...and for the first time understand the "human element" plays a critical role in how colour is perceived.

It's the human eye and the human brain that interpret colour based on the light and the object being viewed. This is studied all the time and the physical elements and the psychological association we as humans have with colour.

The human eye and the way the brain interprets that stimulus is what we has human used to describe colour, with that said we can immediately comprehend that your eyes and my eyes are different, your brain and my brain are different, plus our experiences with colour also effect our Colour Memory.

Light

The LIGHT spectrum consists of the visible range of wavelengths of electromagnetic radiation. Light waves can be altered depending on the medium that the light is passing through, this is known as Refraction.

The Visible Spectrum

Object

When our eyes see colours, they are actually detecting the different wavelengths of the light hitting the retina from the OBJECT. Colours are distinguished by their wavelengths, and the brain processes this information and produces a visual display that we experience as colour.

Viewer

A network of light sensors in our eyes sends electrical signals to our brains. Our brains process these signals into sensations of sight. This means that colours only really exist within the brain of the viewer. The light travelling from objects to our eyes for each object may well be transmitting/reflecting a different set of wavelengths of light or White Light; but what essentially defines a 'colour' as opposed to a 'wavelength' is created within the brain. We use Colour Names to describe how we want to reproduce a given colour. We also run into issues with Colour Blindness when some of the normal retina colour receptors are missing.

The main issues we have with communication colour is describing "How is it represented?" and "How is it reproduced" . The need to communicate colour ties our understanding of colour theory to Colour Management.

The basics of Colour Management require definition of the Colour Model used to numerically describe or graphically plot colours. Some examples of colour models are: RGB, CMYK, LAB HSV (Hue, Sat and Value)

Because colour can be formed using both additive RGB and subtractive CMYK methods, two different definitions, or Colour Space, were developed to describe colour. Colour Space is a variant of the Colour Model with a specific colour range that define the Gamut of the colour space being its' chief character. For example: within the colour model RGB there are many different types, not all RGB is created equal.

RGB Colour - Additive Colour Space

additive colour RGB colour is based on the light spectrum, and it breaks colour down into an RGB representation. In other words, all colour can be defined as a certain amount of R (red), G (green), and B (blue).

Light-emitting devices such as TVs, computer monitors, and so on function in this manner. If you were to turn your monitor off, you would see black because no R, G, or B colours are present. This would be represented as RGB% 0,0,0. If you were looking at a white screen, this would be represented as RGB % 100,100,100 because each red, green, and blue source is shining at full potential. Other colours are created by combining various amounts of R, G, and B. True white light is actually composed of a full spectrum of all light colours, but RGB is close enough for most standards.

Red, Green, and Blue are considered Primary Colours which can't be reproduced by other colours.
RGB is an additive colour space because when you "add" the colours together you get "white".

CMYK Colour - Subtractive Colour Space

CMYK colour is based on colourants and is referred to as a subtractive colour space because you create white by removing all colour. C (cyan), M (magenta), and Y (yellow) are used to create colour in this colour space.

In theory, an equal amount of C, M, and Y would create K (black), but the result in practice is actually a muddy brown. Because of this, K is added to create pure blacks and other dark colours. K is also an economical solution because K ink is less expensive than C, M, or Y.

Because RGB colour spaces are defined by light and not colourants like CMYK, RGB devices generally have a larger colour gamut. This creates some problems because the colour on your monitor (RGB) can be different than what is printed (CMYK).

Cyan, Magenta, Yellow and Black are considered Primary Colours which can't be reproduced by other colours.
CMYK is a subtractive colour space because when you "subtract" the colours together you get "white".

Colour Blindness

We are natural trichromats - we have three different colour receptors that permit us to see a range of colours far broader than many other mammals. The normal human retina's colour receptors (Cones)are tuned to green, blue, and red. Working together, the three give us our colourful view of the world.

When one or more of those colour receptors is missing the result is colour-blindness. The genes for our red and green colour receptors are located on the X-chromosome, giving women a redundant set of receptor genes.

This is why men are far more prone to colour-blindness than women. In order to be functionally colour-blind a woman not only has to be missing a receptor gene on both X-chromosomes, it must be the same gene on each one.

The chances of this happening are so slim that only 0.4% of the US female population is affected. By contrast male colour-blindness is far more prevalent with 8% of the US male population affected - 95% of them with red or green receptor problems.

Colour blindness makes it difficult or impossible to distinguish some colours, depending on which receptor is affected. The term colour-blindness itself is somewhat of a misnomer, since colour perception is altered, not eliminated. True colour-blindness, wherein a person can distinguish no colour at all, requires a malfunction of all three kinds of colour receptors, and affects only 0.003% of the population regardless of gender.

Various Tests for Colour Blindness

Colour blindness is the inability to distinguish the differences between certain colours. The most common type is red-green colour blindness, where red and green are seen as the same colour.

Usually Isihara (pseudoisochromatic) plates are used to test colour vision. They are made of dot patterns composed of primary colours. These dot patterns represent a symbol that is superimposed on a background of randomly mixed colours. The test can determine certain abnormalities in a person's colour vision.

Colour Memory

Our colour memory and what we refer to as "Memory Colours" contribute to the challenge of Colour Theory and Colour Management.

Memory Colours are colours matter more to us because we have strong memory of them. Colour is a property of light while memory is a property of our experiences. These are colours such as skin tones, green grass, or sky blue that we are all familiar with. Some colours are more important to get "right" and out ingrained memories of these colour are often quite inaccurate. There are psychological aspects of the human colour perception that we can't yet model mathematically, so colour management simply can't address them. Even the best colour management must leave room for human intervention at strategic points.

Then we have the human mind, our experiences and our perception. Take a moment, close your eyes, think of a green grassy meadow. Can you see it in your minds eye? I know what I see, you know what that looks like, but if we were actually about to measure the colour in our minds eye-they would be different due to your brains interpretation.

The human mind is adaptable, if we talk about red roses, we've all seen red roses but we know that not all red roses are the same. Our mind knows that it's in a range and when we close our eyes our mind remembers from our experiences what red roses look like.

Yellows are very sensitive to us, and it's hard for the human eye to distinguish ranges of yellow when we look at a gradient.

Here's another example, an artist will spend years painting, with an unlimited number of custom paints he can mix on a whim, to reproduce an image he has in his minds eye. When he's happy with it, he brings it to you and wants you to scan it and print it using 6 colours and make it look like he thinks it should. This is why we use Colour Names.

Colour Names

When we use colour names, we set a minimum bar for reproduction quality. We define Hue as the attribute of a colour by which it gets its basic name. The connection between hue and basic names isn't just a philosophical nuance, but may be one of the most important things to remember about colour reproduction. Generally we are fussier about discrepancies in hue that we are when it comes to brightness or saturation between a target colour and its reproduction, or between display colour and its print.

If the hue is different enough to cross some intangible boundary between colour names - such as when your reds cross slightly into the oranges - then people notice a hue shift.

Human Eye

CORNEA - The transparent "front window" of the eye. It is a thick, nearly circular structure covering the lens. The cornea is an important part of the focusing system of the eye.

PUPIL - The round black hole in the center of the iris. The size of the pupil changes automatically to control the amount of light entering the eye.

IRIS - The pigmented (coloured) membrane of the eye, the iris is located between the cornea and the lens. Its colour varies from pale blue to dark brown.

LENS - A transparent biconvex structure located behind the iris. It focuses light rays entering through the pupil in order to form an image on the retina.

RETINA - A thin multi-layered membrane which lines the inside back two-thirds of the eye. It is composed of millions of visual cells and it is connected by the optic nerve to the brain. The retina receives light and sends electrical impulses to the brain that result in sight. Rods and Cones are the cells in the retina that respond to light.

MACULA - An area of the eye near the centre of the retina where visual perception is most acute. The macula is responsible for the sharp, straight-ahead vision that is used for seeing fine detail, reading, driving, and recognizing faces. It is one hundred times more sensitive to detail than the peripheral retina. The macula is sometimes referred to as "the bull's eye centre of the retina."

OPTIC NERVE - Cable-like structure composed of thousands of nerve fibres that connect the macula and retina with the brain. The optic nerve carries electrical impulses from the macula and retina to the processing centre of the brain where they are interpreted into clear, colourful sight.

Rods

Rod cells, or rods are photoreceptor cells in the retina of the eye that can function in less intense light. Since they are more light sensitive, rods are responsible for night vision, but also allow for the perception of colour. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. There are about 120 million rod cells in the human retina.

Cones

Cone cells, or cones are cells in the retina of the eye which only function in relatively bright light. There are about 6 million in the human eye. Cone cells are less sensitive to light that the rod cells.

The human eye has three types of colour sensors, or cone cells (corresponding to the red, green, and blue) is what lets us reproduce colours at all using just three pigments on paper, or just three phosphors in a monitor.

Thus the "colour" of any light can be specified by three numbers (e.g. the photons caught by each cone type).

The first type responds most to light of long wavelengths, peaking in the yellow region; this type is designated L for long.

The second type responds most to light of medium-wavelength, peaking at green, and is abbreviated M for medium.

The third type responds most to short-wavelength light, of a violet colour, and is designated S for short. The three types have peak wavelengths near 564-580 nm, 534-545 nm, and 420-440 nm, respectively. The difference in the signals received from the three cone types allows the brain to perceive all possible colours, through the opponent process of colour vision.

Refraction

By definition, refraction is a change in the direction and speed of a light wave passing from one medium to another. As a ray of light passes between the boundaries of the two media, it changes direction and its wavelength can increase or decrease depending on the properties of the two medias.

For example, glass has a higher rate of refraction than air. A ray of light passing through the boundary between air and glass will have its speed and wavelength altered. This effect can be seen visibly when a ray of white light passes through a glass medium. The different waves of light are spread out as they pass. The result is a prism of light.

For profiling clear media, such as glass, refraction can compromise the ability of the colourimeter to give accurate readings as the light leaving the colourimeter is changed when it passes between the boundaries of one media to another. The light scatters instead of reflecting back unaltered.

White Light

A ray of white light is composed of a myriad of light waves with wavelengths that encompass the visible range. When a ray of white light strikes an object, many of those waves are absorbed. When the remaining waves of light strike the human eye, they comprise the colour we perceive the object to have.