How can red plus green make yellow?

*Question *

**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.
**

*Answer *

**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*