Rust (brown) mixed with green can look that way when draining. I bought my truck from a dealer and i know it had a leaking freeze plug that either the previous owner or the dealer put in stop leak. After i got it home i pulled the lower freeze plugs on both sides of the block. I then put some brass fittings together(so i could position the nozzle) to pressure wash out the block. You would be amazed at the curd at the bottom of the block. Since this is a Texas truck it may have been run with out antifreeze at some point. This may be something you could consider if you see rust coming out when you flush it.