See https://www.nytimes.com/2017/03/31/us/politics/william-coleman-jr-dies.html for his obituary. The most plausible way I can see him getting on the US Supreme Court is if Gerald Ford wins in 1976 and Potter Stewart decides to retire a few years earlier than in OTL. After 1976, Ford is not eligible for another presidential term, so he does not have to worry too much about possible conservative opposition to Coleman. OTOH, he may want to appoint the first woman to the Supreme Court--appointing an African American will be less novel...