Anti Freeze turned brown.

First off, I wouldn't use 50/50. Definatley do not use Dexcool, extended life, or mixes with everything anti freeze. Remove the block drain plug on the passenger side of the engine, just to the rear of the oil pump area. Poke a screwdriver or something into the hole, to get the water flowing out. Flush the system with at least water to get the block somewhat clean.
What I do every fall, since My race car(s) get only plain water all race season, is with the thermostat out, I add 3 ozs of swimming poll (muriatic)acid mixed with about a pint of water to the cooling system (that is almost filled with plain water). Let that solution circulate for about 1/2 hour, and drain completely (radiator and block). Flush, flush, flush, with plain water. Then add full straingth "green" antifreeze.