could be a few things wrong. the tranny fluid could be leaking in, or there cuold be motor oil gettig into the water somehow. This could be caused by a blown head gasket or cracked head.
Take it to a radiator place and ask for a pressure test....there should be a slight brown slick in the coolant as they put in anti leak blocks aswell when they change coolant as a precaution... but if it's full blown brown... then get it checked out.