There are several different colours possible in the Boston Terrier breed as far as I'm aware (red/brown/chocolate, blue, fawn, black, brindle, seal), but only Black, Brindle & Seal are acceptable colours according to the standard.
What Kade said about a breeder looking at the dog as a whole rather than an individual trait is true and you'll find most, if not all, reputable breeders will not breed the colours that are considered a 'fault'.
You could contact a few rep. breeders and ask if the colours you're interested in ever just 'pop up' in a litter though. Or look for a rescue Boston with the same colour.
I think, though, that you'll find it's byb and puppymillers who tend to turn out the majority of these dogs. They concentrate on colour and forget everything else.
Which is sad in a way (I understand why reputable breeders don't breed them though), because I love all the different colours I've seen Bostons in, too.
Cass.