I think Colombian seems to fit. The guy who sold him to me assured me he was pure banty Cochin. But again I am no expert. I should also say he was free, the guy had way too many roosters. But he had 3 like this one, a few grey and a few white.
Just because he's not a standardized/accepted color doesn't mean he's not a purebred. Just means that someone took the time to create a new color in the breed and it wasn't as popular as others. For instance if I were to create this color in cochins I'd cross it with a Buff Colombian, keep the chicks that look most like the cochin breed standard and breed them back to more cochins. Over several generations the Brahma would be essentially bred out except for the color. I am not sure how many generations this takes in chickens but in dogs and cats the accepted amount to be "pure breed" again is three generations away from the original cross. It takes time but it's a fun little project. Someday I hope to do this to make chocolate Brahmas. Need to move to a bigger acreage first though!