Likely nothing. White people didn't face legal repercussions for using "colored" fountains. There would have been a chance of social derision from the White community, due to the association with African Americans, or the significantly lower quality of "Colored" specific areas but nothing illegal would have happened if a white man used a black restroom or water fountain. There are however instances of white on white violence regarding Civil Rights such as the beating of Jim Peck for his involvement in the 1961 Freedom Rides, and the murders of William Lewis Moore in 1963 and Viola Liuzzo in 1965 both for working with Civil Rights activists.