Brad DeLong writes that he

Brad
DeLong
writes that he only just realised that there could be non-spectral
colours.

Until yesterday, it had never occurred to me that I could see colors that
weren’t in the spectrum–I had thought that all colors were somewhere in the
rainbow (or could be made from rainbow colors by darkening or lightening them).

But that is clearly false. Consider magenta. A magenta light plus a
green light equals a white light–all colors. But green is in the middle of the
spectrum. So where in the spectrum is magenta? Magenta is red and blue–the
complement of green. And nowhere in the spectrum is there a wavelength of light
that excites both the red-cones and the blue-cones but does not excite the
green-cones.

I was
going to write a comment saying just how magenta was possible, then I realised
I wasn’t exactly sure. Then I was going to link to a website that explained it
all clearly, until I realised I couldn’t find one. So if anyone could enlighten
me, or Brad, please write in!

Here’s
what I think happens, though I’m not entirely sure. The spectral colours are
colours produced by light of constant length. But we know there’s lots of waves
that do not have constant wave lengths. This is obvious for sound: you never
hear the sound of a trumpet, even a trumpet playing a ‘constant’ note, when you
just listen to waves of constant length. Magenta, I think, is one of the things
that happens when the light in question is not a wave of constant frequency.

But, that
doesn’t really say enough about what happens. I don’t know how the waves ‘mix’.
Is it that magenta light contains only photons of a constant frequency, but
some of them are around the typical frequency of red light and some of them
around the typical frequency of blue light? Or is it that individual photons
‘vibrate’ in some non-sinusoidal pattern, as the air does when two or more
notes are played? Or does this distinction not really make sense when we’re
dealing with light?

And
I’m not even sure this is the right story about magenta. I think it is, but for
all I’m certain of, magenta could be a contrast colour, like brown, that is
only apparent when there are other visible colours with which it contrasts.

Some
might think that it’s embarrassing how little I know about colours, but (a) if
I was going to be embarrassed by my ignorance there are many other things I’d
be embarrassed about first, and (b) since my department already has an expert on
colour
, the marginal value of my learning more is not very high.