In the olden days of semi-free markets, some government contractors were hiring black people since they were cheaper than white people and just as effective. The work got done for cheap, but just think of the social consequences!
Senators Davis and Bacon wanted to prevent this kind of race to the bottom, so they passed the Davis Bacon act. Davis Bacon is still on the books and massively drives up the cost of many projects.
According to the wiki, it was discriminatory at the time because it generally meant that only union workers would get federal construction jobs, and unions did not admit African-Americans. As this is no longer the case with unions, it should in theory no longer be discriminatory - just wasteful.
What do you mean by white people laws?