Colour

Colour is slippery

It is very hard to talk about colour. When I try to be specific and accurate I soon realise the words I want to use don't quite mean what I thought they did. They are used for different things in different situations. Hue is not a synonym for colour nor is brightness a synonym for lightness, luminance or value (despite the assertions of my thesaurus).

And if at any time I speak of Light and Rays as coloured or endued with Colours, I would be understood to speak not philosophically and properly, but grossly, and accordingly to such Conceptions as vulgar People in seeing all these Experiments would be apt to frame. For the Rays to speak properly are not coloured. Isaac Newton - Opticks

Also this page is still a stub. It will hopefully be completed soon.

Light

Light doesn't so much have a colour as much as it has a wavelength.

Colour is what we perceive when that light excites our eyes. It is our minds interpretation of those wavelengths.

There are three types of colour receptor cells in the eye, the cone cells. Their sensitivity ranges overlap so light of a single wavelength may trigger more than one type of cone cell. The hue perceived is determined by the ratio of the types that are triggered.

When light of wavelengths near 589nm come into our eye we might say:

See that sodium lamp? That's yellow.

But there are also mixtures of green (520–560nm) and red (700–635nm) lights that can cause exactly the same response.

RGB pixels
Tricolour television close up
Monitor 1 No yellow pixels here.
See that particular mixture of red pixels and green pixels on the TV? That's yellow too.

This phenomenon of multiple different possible causes triggering the same response is called metamerism.

More bizarrely we can say:

See that mixture of red and blue? That's magenta.

The perception of magenta cannot be produced by light of any single wavelength. Magenta isn't anywhere on the spectrum of visible light. It is just the perception in our head produced from certain ratios of wavelengths.

This is the sort of thing that leads some people to say:

Light doesn't actually have colour. Objects don't have colour either.

If there's enough different wavelengths and they are sufficiently evenly spread…

See that mixture of assorted wavelengths? That's white.

It's important to point note that white is extra-spectral. There is no white wavelength. There are no white photons. It is always a mixture of other wavelengths.

And finally:

See that mixture of no light? No? Neither do I. That's black.

The dimensions of colour

The CIE defines a colour space as a geometric representation of colour in space, usually of 3 dimensions. CIE 17-22-059 colour space

Physical colour models

Physical models often use dimensions which suggest how the colour could be constructed.


Perceptual colour models

Perceptual models use dimensions that relate more to the human vision. They try to model the way in which the colour is perceived rather than how it's created.

Hue Represented by angles around the central value axis.

The closest colour on the colour-wheel.

Hue is determined by the ratios of the firing rates of the different types of cone cells.

Value, brightness, lightness or tone Represented by distance vertically parallel to the central axis.

A ranking of the amount of light present.

It is determined by the total combined excitation levels of all types receptor cell.

Brightness is our perception of luminance (visible light energy),

Lightness indicates how dark or light a colour is.

Saturation, chroma, intensity or colourfulness

Saturation indicates colour strength, the purity or intensity of a colour.


Chromaticity


Chromaticity diagram: an xy slice though CIExyY

Chromaticity is an indication of hue and saturation without brightness. It provides a qualitative description of a colour without mention of its value.

In the chromaticity diagram we see a horseshoe shaped area which represents the gamut of human vision. The curved locus at the top is where the pure single wavelength colours would lie. The straight line at the bottom is the line of purples made from fully saturated extra-spectral colours. The triangle in the middle shows the colours that can be encoded in the RGB space and represented on a computer screen.


Perceptually uniform colour spaces

A perceptually uniform colour space attempts to make the difference between two colours (as perceived by the human eye) proportional to the Euclidian distance between the points within the space. CIELAB does a fair job in any local area but not so well as the distances increase.

Over the years the CIE has defined improved colour difference distance measures (Delta E or ΔE*ab) to help: CIE76, CIE94, CIEDE2000

Things

Objects reflect some, but not all, of the light that falls on them. The biases in the reflectances of different wavelengths are an objects colour reflectance spectrum.

If an object under white light reflects only the yellow wavelengths I'd call it a yellow object.

We can make a paint that reflects red light and green light but absorbs all light of other wavelengths. If we paint a car with it and put it out in the sun I'd call it a yellow car. But if we shine cyan light at it, made from a mix of blue and green light, only the green will bounce off it and the blue will be absorbed. It will look completely green. If it's indistinguishable from another green car is it still a yellow car?

Remember, a cyan Volvo under a yellow sodium street light against a purple background is going to look…   ugly.

Paint

We have talked a little about mixtures of light, now let's consider paint.

Like any object, a paint's colour is determined by what wavelengths it absorbs and what it reflects. When paints are mixed their colour reflectance spectrums are combined.

Yellow and blue paints makes green (sometimes)

Most paints reflect a wider range of wavelengths than we'd expect just from looking at them. A yellow paint may well be reflecting many orange and green wavelengths but the sensation is still yellow. Similarly a particular blue paint may also be reflecting some green wavelengths and others. When we mix them only the wavelengths that both paints have in common are reflected. In this case green.

   ROYGBIV
   _oYg___   a wide spectrum yellow
 + ___gBi_   a wide spectrum blue
 = ___g___   green: the only commonly reflected wavelengths
 

Note that other, similar looking, yellow and blue paints may not mix the same.

   ROYGBIV
   __Y____   a narrow spectrum yellow
 + ____B__   a narrow spectrum blue
 = _______   black: no commonly reflected wavelengths
 

Some people like to mix their own blacks but they're never as dark as ivory black or mars black. So don't bother.

   ROYGBIV
   Ro____v   red, but this particular red paint also reflects a little orange and violet
 + __y_B_v   blue, but this particular blue paint also reflects a little yellow and violet
 = ______v   black, but not quite
 

A colour can have any number of metamers produced by different spectral powder distributions. Different paints of the exact same colour may have very different mixing characteristics. In general you can't tell a paints mixing characteristics by its colour (in any given light).

What we can know is that when we vary the ratio of a mixture of two paints we will produce a continuous path between them through a colour space. The path of mixture will certainly pass through all intermediate values of hue, values and saturation. This gives us a nice handle we can use.

Further notes on paint mixing can be found in Tutorial 1 Part 4 - Colour mixing.