There are lots of reasons why two employees might make different salaries and race isn't one of them.
I don't know how old you are, but I when I entered the work force as a young woman, it was standard and acceptable for a woman to make substantially less than a man doing the exact same job because women were making "pocket money," while their husbands supported the family.
We have a small seafood processing plant here that only employs Hispanics (although all the supervisors are white) and even provides some of them inexpensive housing so they'll stay. I'm sure they pay a much lower hourly rate to these green card workers than us white folk would consider acceptable.
If your business is actually fair, good for you!