Where is colour mixing?

Imagine that we have three projection lamps at the back of a hall – one has a red filter and so produces a beam of red light, and the other two use filters to produce green and blue beams. We project these onto a white screen and get three circles of light (one, red, one green and one blue). We then move the angles of the projectors so that the circles of light overlap. We get something that looks rather like this:


Where the red and green light overlap we get yellow. We get magenta and cyan for the other two binary mixtures. So,

red + green = yellow

red + blue = magenta

green + blue = cyan

This is called additive colour mixing as I am sure you know. And if we mix all three primaries we can achieve white (or other neutral colours). The primaries could be single wavelengths of light – so we could use a primary at, say, 700 nm (for the red) and one at 450 nm (blue) and one at 530 nm (green). So green light (530 nm) and red light (700 nm) additively mix together and generate yellow. When this happens what is being mixed and where does this mixing take place? Take a few moments to consider this before reading on.

Notice I said that they additively mix to generate yellow – I specifically avoided saying that they mix to generate yellow light. When I sat down with a couple of students last week and asked then what they though they said that the red and green light mixed together to create yellow light and when I pressed them, they went further to say that the yellow light was at about 575 nm.


If we measure the part of the screen that is yellow we would see that we have some light at 700 nm and some at 530 nm. The wavelengths are not mixed; they don’t mix together to generate some third wavelength of light such as 575 nm. So no physical mixing takes place other than – I suppose one could argue – that the red and green lights are mixed in the sense that they are spatially coincident. But that’s not really mixing, for me, and certainly doesn’t even begin to explain why we have the sensation of yellow when we look at these wavelengths together. It also makes me think that additive colour mixing, if it can be said to occur anywhere in particular, occurs in the eye. And I do mean eye, not brain.

2 thoughts on “Where is colour mixing?

  1. Steve, I concur with the view that colours as such don’t exist to be mixed outside the brain, and hence that “additive colour mixing” is strictly speaking a misnomer, and really means additive colour-stimulus mixing. A “red-making” and a “green-making” light mix on the wall and send to the eye a light mixture that most likely includes wavelengths that separately would be orange, yellow and cyan, as well as red and green, since the original lights were not monochromatic. Though made in this case FROM a red and a green light, this stimulus mixture is not inherently made OF two red and green components; it could be split up into five spectral components, or fifty, and recombined to make an identical stimulus.

    According to the generally accepted model, each cone type responds separately to the balance of wavelengths in the spectrum of this mixed light stimulus. What are “mixed” in the eye are these three cone repsponses, not “red” and “green”. Colour itself doesn’t enter the picture until the brain creates it based indirectly on these mixtures of cone respones (the cone-opponent signals). Based on this particular combination the brain creates a strong yellow colour-opponent signal and a red/green colour-opponent signal near zero. Strictly speaking the only colours that actually mix, ever, are these red/green and yellow/blue colour-opponent signals created by the brain.

  2. I totally agree David. I gave the example of single-wavelength red and green for simplicity. In practice, as you say, the situation is likely to be more complex since most stimuli are not single wavelength. Nevertheless, the principle is the same.

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