Colors are interesting since there are groups of wavelengths we call a color, and even if you narrow it down to one specific wavelength, people may perceive it differently. And it's hard to know whether we perceive it differently because we're actually seeing something different, or because of some kind of linguistic determinism--there are studies that show that for whatever language doesn't have a name for a particular color, the people who speak that language also have trouble differentiating that color from a color like it.

But yeah, the dress thing is another aspect to it as well.