In federal prisons, at least, the population is about 59% white, 37% black. Almost 35% are Hispanic, which could be of either race. 76% are US citizens, 17% Mexico, 7% other countries (of which Latin American countries are probably a solid majority).

http://www.bop.gov/about/statistics/statistics_inmate_citizenship.jsp

http://www.bop.gov/about/statistics/statistics_inmate_ethnicity.jsp

http://www.bop.gov/about/statistics/statistics_inmate_race.jsp