There can be the other way:
When I paint my worlds I'm creating some temporary palettes (Swatches in PS). I can add my RGB colors and give names to them in it. PS allows to export(/import) swatches in several formats, like .ase.
Maybe add feature into TC to optionally allow loading .ase file (setup it's filename in world config). TC should parse it, get names, interpret them as names of preloaded biomes, and use that colors instead of that set in biome configs.
Working on map:
If TC founds biome with such name them use color from palette otherwise use color from biome config.
I don't know what is the most popular format for palettes, .ase looks like as quite simple. We have to look what are other formats and what programs supports most of them. Maybe support several formats...