consult an attorney on legality. Only an attorney is allowed to give legal advice.
If the staffing agency knew of the wrongful reason, and because of it (or because of the other company's decision to end your assignment with knowledge of it) they then declined to assign you other work, you might have a case for wrongful termination case against them (by way of constructive dismissal), but there would seem to be a number of things you'd have to prove there that you wouldn't in a direct employment wrongful termination case.
IANAL, but my understanding is that judges pretty quickly look past what things are labelled. If the temp was able to quickly roll into another gig (or the temp agency otherwise kept paying them), that's one thing. But I'd still be very surprised if legal cleared "We don't have need for your services any longer because you're black. We're contacting your temp agency for a white replacement."