E.g. Jim Crow...
Edit: I'm not looking for anything really specific. Just a few examples of cases where federal courts overturned state laws they found to be discriminatory, just for my own information. Thanks!
I am confused by the way you phrased your question. Federal courts are always justified in overturning state law because whether or not a law should be upheld is one of the main things they sit and decide, and the supremacy clause of the US constitution means that such decisions are well within the purview of the federal courts. Could you rephrase?