Is your thermostat still there? If yes then your method of flushing isn't going to work. The thermostat won't open if it's that cold (your running water) so the water in your block won't flow out. The rusty colour is probably from your block. The best way to flush is to buy the thermostat housing gasket (if your car needs one), take out the thermostat, then only stick the hose in to flush out the milo. Check your hoses also to see if any of the crud is gunked up inside the hoses and use a bottle brush or something to scrub it clean. Then after you flush all the rust out fill with distilled water/RO Water which u can easily buy in 5L bottles in Giant or somewhere. Mix it with concentrate coolant 50-50 sorta kinda or just fill with premix coolant only. This kinda cleaning should last you minimally 100k km.