Anti Freeze turned brown.

I use a bottle of fast flush from Napa and run it up to temp so the t=stat opens then drive it a few blocks, shut it off and flush it with a t adaptor in the heater hose until it comes out clean then refill with water and repeat flush again if it is rusty brown. They usually come clean after 2 in a row and stay clean with a drain and fill every 2 years.