So I've got a 2008 Impreza Hatch with a 2.0l EJ204, had it for 4 years and I just realized today that it had the wrong type of coolant since I've got it, G12 red coolant, and it's supposed to use G11 Blue/Green Coolant. This may answer some questions as why my water pump started leaking after 32k miles, Why I had to replace 3 hoses and the thermostat seal last summer and so on. I now need to replace the timing belt and all the pulleys with the water pump so I will be draining the coolant once again, my question is: Is draining the coolant from the engine and pouring like 10 liters of whater in for it to flush going to be enough? I really do not want to mix 2 different types of coolant after it's been running the wrong one for so long. 

Just stick the garden hose in for a minute if you can, with the drain screw open. Flush it until it runs our clear.

Is it necessary to flush radiator if the new coolant is different colour?