RedShark Replay: If you don't understand how we see colour, you don't understand colour. This may be the most important lesson about colour you ever read (first published October 2014).
There is no such thing as colour, at least in optical physics. Colour is a biological, or even sociological construct. Witness the fact that azure is a shade of blue, but calling azure “blue” in Russian or Italian is as wrong as calling pink “red” in English. But beyond the anthropology of it, anyone who's spent any time behind a camera will be familiar with the problems associated with rendering colours in an acceptable, not to say realistic, manner.
Given that we're taking a spectrum of effectively infinite colour and recording it as a proportion of three red, green and blue primaries, we probably shouldn't be surprised that things sometimes go awry. As the existence of colour rendering indices attest, there's even some concern over creating a reliable and repeatable white light. Even colourlessness is not straightforward.
What's key to all this is the somewhat shaky idea that our eyes, unlike camera equipment, are reliable arbiters of what the world looks like. Even overlooking the somewhat existential concerns about what “red” looks like to different people, it isn't necessarily the case that the human visual system is entirely reliable in matters of colorimetry.
Like cameras, our eyes are at least to some extent tri-stimulus devices, with long (reddish), medium (greenish) and short (bluish) light-sensitive elements. These are fairly unsaturated, with large overlaps, and the green and blue are generally quite close together. Colour vision is therefore quite reliant on what a video engineer would call matrixing, the extraction of extra colour information based on the differences between the three channels as well as their absolute values.
As such, our eyes may be fooled by emitted or reflected light with specific colour properties in much the same way as a camera. While human vision may, through necessity, be our model for ideal camera performance, there's certainly nothing particularly reliable about it as regards the perception of colour.
Simultaneously, the biggest problem and the greatest advantage of this situation is metamerism, the phenomenon by which light with a particular spectrum may appear to some detectors (such as our eyes or a camera) to be identical to light with a different spectrum. Metamerism is a subtle and complicated issue which is both the basis of almost all colour imaging and the source of many of the problems with discontinuous-spectrum light sources such as LEDs and fluorescent tubes.
The problem in detail
Let's look at a simple example. In the figure below, the left-hand chart shows a spectral power distribution – that is, the amount of light at different points on the visible spectrum - for a light source. This light source contains energy at a wide variety of wavelenths, from blue to red with a peak in the blue. The right-hand chart shows a different spectral power distribution including only narrow bands of emission in the red, green and blue, with the blue peak higher.
While the mathematics are a little idealised for clarity here, it's quite possible that these two light sources would appear identical – a fairly unsaturated purplish-white. This is, of course, the basis behind which almost all colour imaging operates. A video display cannot actually emit (for instance) orange light, it can only emit red light, and add green to it, such that the result (right) appears to match the orange (left) as photographed: