[equil]

"[...] but since the color spectrum fits on a single line [...]"

There's a lot I want to respond to that statement with (purple!), but I'll just mention that photons have more than a single frequency parameter [1]. Contrary to popular belief, this is relevant for human color perception, as the cone photoreceptor cells are sensitive enough to polarization that it can be perceived under certain circumstances [2] (not to mention the impact it has on light intensity in for example gemstones through fresnel effects.

I'll just leave this here: http://makeartbutton.com/dmp/lusion.png

[1] https://en.wikipedia.org/wiki/Stokes_parameters

[2] https://en.wikipedia.org/wiki/Haidinger%27s_brush