How can red plus green make yellow?
By adding the three primaries of a computer screen in various proportions all hues should be obtainable. The primaries are red (R), green (G) and blue (B). It is obvious that R and B can give blue-red hues, such as violet, magenta, purple. It is also obvious that G and B can give turquoise, which is a blue-green hue. But how can R and G give yellow? Yellow is not sensed as a green-red hue.
It is not as mysterious as it might seem. By closer scrutiny you will find that R is a yellow-red colour, and that G is a yellow-green colour. So, when these lights are added together the red and the green aspects of the colours balance out each other, being incompatible. What we see is the remaining yellow. It was there already from the beginning, so to speak!
You can nicely demonstrate this on your data screen with a suitable computer program. First you show the pure primaries R and G, overlapping to make Y (yellow). Then you add into R a suitable amount of the primary B to make it look just red, i.e. a red which tends neither to yellow nor to blue. Likewise, you add into G such an amount of B that it looks neither yellow-green nor blue-green, but just green. (Se illustration below) Then the two colour fields do not any longer add to make yellow, but rather white. They are complementary. The hues neutralize each other, with an achromatic result.
At least, so it is according to the opponent colours theory. Empirically it is not perfectly, only approximately true but it is a sufficiently close approximation to be enlightening and useful!
© Pehr Sällström, febr. 2006