my reasoning: the actual colors we can see -> the wavelengths that we can extrapolate to -> basically extrapolated wavelengths plus an 'unpure-ness' factor -> not even real wavelengths (ok well king blue and maybe lavender if I'm being generous could be)
Wikipedia calls metallic colors colors, which implies that any arbitrary brdf is now a color. Personally I would only call the photons travelling through the air at any specific point in space a color, and say that the object's color changes when you move it around. I guess you would still say that a red metal sphere is red though, so my definition doesn't really work.