In Texas, the vast, overwhelming majority of employment relationships are "at will," meaning there is no guarantee that the employer will continue providing a job or that the employee will continue working. To the contrary, in "at will" employment, either side can terminate the employment relationship at any time, with or without advance notice, and with or without good cause, or any cause whatsoever.
There are a very few exceptions. At least in some circumstances, you can't be terminated as retaliation for bringing a worker's compensation or certain other kinds of legal claims against an employer, or for refusing to do an illegal act. There is a chance that your facts could squeeze you in under that second theory. But to prove your case, you'd need to prove both that the employer was illegally discriminating on the basis of race with respect to the other employees, and then that he was indeed basing his decision to fire you on your refusal to participate in that course of action.
Such cases are very difficult to prove. Besides having to prove what's essentially a wicked intent on the part of your employer -- both as to you and as to the others -- you are going to have a hard time showing a large amount of lost wages as your damages, because you had no promise or guarantee that you wouldn't be let go at any time for some reason OTHER than refusing to violate the law.
Unless you had some documentation that supported your position, or several other witnesses (besides the others who were fired) who would support your testimony, the odds of winning this sort of case are very grim. It's unlikely that you're going to be able to find a good attorney to represent you on a contingent-fee basis, which would mean you'd have to pay someone by the hour to represent you -- which would quickly run into the thousands, even tens of thousands, of dollars.
I don't want to discourage you from consulting with attorneys -- there are some fine ones in the Dallas area who are very creative, and many of them offer free initial consultations -- and it's conceivable that in listening to your full story, they'll spot something I've missed. But I don't want to give you false hope. Without a written contract guaranteeing a particular term of employment, it's very, very difficult to win a wrongful termination case in Texas.
By "consulting with attorneys" in my original answer, I meant, of course, consulting with attorneys on a face-to-face basis in an attorney-client relationship -- something that's lacking from any general and casual advice given via an internet website like this one.