I tried adding purple to the project color data, but I got this response:
message: color: Must be one of {dark-blue, dark-brown, dark-green, dark-orange, dark-pink, dark-purple, dark-red, dark-teal, dark-warm-gray, light-blue, light-green, light-orange, light-pink, light-purple, light-red, light-teal, light-warm-gray, light-yellow, none}, not: purple
So at least I know what colors I can use now.