Weigh the Balls :)

Given 3 red and 3 black balls. The 3 red balls weigh different, one heavy, one medium and other light. Similar is the case with black balls. The heavy, medium and light balls of both colors weigh the same. One heavy and one light ball weigh the same as two medium balls. Given a weighing balance, in 4 weighs, categorize each ball of both colors as heavy, medium and light.