Inverted triangles of different colors were used by the Nazis to identify concentration camp and death camp prisoners' race/religion/arrest status.

Yellow Triangle: Jew
Red Triangle: Political prisoner (if the triangle pointed up it denoted a German political prisoner)
Pink Triangle: Homosexual
Green Triangle: Criminal
Black Triangle: Gypsies and "asocials" (mostly the homeless and prostitutes)
Blue Triangle: Emigrants
Purple Triangle: Jehovah's Witnesses

A letter in the middle of the triangle identified the person's nationality. For example, a T meant Czech, B meant Belgium, F meant France, and an I meant Italy.