As a postscript to the discussion of Rainbow<->Xeno color mapping a
couple of months ago, I proposed a set of colors to maintain the warm/
cool/light/dark/opaque pattern I pointed out.
Warm:Magenta Cool:Aqua Light:Pink Dark:Brown Opaque:Grey
If it weren't for brown, I might call it the "Pastel" set.
Riffing on Jeff's light/dark train of thought, that gives pairs for
many of the colors: white/black, pink/red, aqua/green, cyan/blue,
yellow/brown, and if you stretch a bit, magenta/purple.
The leftovers are clear, opaque grey, and orange. I think an even
more hypothetical fourth Treehouse set should try to pair up with
those three, adding another light/dark pair to fill out the set of
five. A dark version of clear would obviously be "smoke". Grey is
problematic, being both "light black" and "dark white"
simultaneously, and most likely a different shade of opaque grey
would be required. Orange too is a very medium shade -- darken it
much and it gets close to red, lighten it and it approaches yellow.
I think one might be able to make it just dark enough to produce a
distinct "pumpkin" shade. What's left in the spectrum is the puce/
lavender range, and I think one might be able to find a deep violet
that is distinct from purple, and a light mauve that is distinct from
Warm:Pumpkin Cool:Mauve Light:Smoke Dark:Violet Opaque:OtherGrey