Basically, green to green, red to red (orange to orange/red). Mixing traditional green coolant with the traditional long life red coolant will result in a gel inside your cooling system, so never mix those two coolant types. Now that being said, most of what is out there today is able to mix with either type, just make sure to look at the bottle. If it says "mixes with any type, any color", then you're good to go. If it doesn't, then make sure you put in the correct type or pick up a bottle that says mixes with any type any color.