I recently posted a somewhat sideways thought
to twitter. Expanding the thought properly requires more than 140 characters, so I'll do it here rather than there.Purple number one: Violet
Imagine looking at a rainbow. (If a real rainbow is available, stop wasting time reading this and go and look at it. Admiring pretty things is a vital and shockingly undervalued lifeskill.) Purple is the bit between blue and ultraviolet - since most people can't see ultraviolet, it is therefore the innermost chunk of the rainbow, fading into blue on one side, and invisibility on the other. This type of purple consists of photons with wavelengths in the rough range 380 to 450 nanometres.Purple number two: Magenta
Now imagine looking at an LCD screen showing a picture of a rainbow. (Feel free to find one on whatever device you're using to read this if you've not practiced admiring pretty things recently.) It looks very similar, right? But your screen isn't emitting a single photon in the 380-450nm range; it isn't physically capable of doing so.
So far, this is actually entirely normal, as far as human vision goes. (Human vision, like almost everything else to do with the human nervous system, is a weird heap of barely functional kludges piled on top of other, older, kludges in a way that really shouldn't work but collapses surprisingly infrequently for something so absurdly incoherent. But that's another story.) Most colours can exist in either of these forms. So a yellow flower might be reflecting just a single wavelength in the 570-590nm range, or it might be a mixture of different wavelengths, some longer and some shorter, the average of which lies in that range.
And that's where purple gets unusually
weird. If you want to show cyan, you mix green (longer wavelength) and blue (shorter wavelength), and your eye/brain interprets this as being something between the two. But if you want to display purple, you mix blue (longer wavelength than purple) and red (much
longer wavelength). And then your brain says "halfway between red and blue is green, but there isn't any green here, so it must be something else", and shows you purple. Some of the magenta-purples you see when it does this are indistinguishable from the violet-purples that you see looking at a real rainbow. But it's possible to combine reds and blues to produce magenta-purples that (to a typical person's perception) don't match any wavelength of light at all.
There are other types of colour that can never be produced with a single wavelength of light - brown is the obvious example - but "Purple" is unusual, in English at least, in being both a chunk of the visible spectrum and
a batch of non-spectrum colours.
 in practice, a flower is never going to be reflecting just a single wavelength. In fact, even sodium streetlights emit two, albiet two pretty close together. But imagine a flower that does, OK?