Why are my Terraria characters restricted from being certain colors?

I believe the idea is to prevent people from making characters that are too dark. If you could turn yourself too dark you'd be invisible in some areas of the game, which would give you an unfair advantage in PvP.

There's a thread about this on the official Terraria forums.

Even as far as workarounds go, I expect the best you'll be able to do is to scale your color up. 3B6363 (59, 99, 99) is the closest color which Terraria will accept, which is at least still nicely dark.

Terraria InvEdit allows all colors to be accessed, you can also change your name.