Just a cautionary note - there is no "standardised" colour scheme for coolants so be wary of going by colour to determine what type is in there, or what type you're putting in. Your green coolant could well be a "long-life" coolant anyway.
There are three main types of coolant:
Inorganic Acid Technology (IAT) - Older type coolant which is often (but not always) blue or green and relies on silicates to coat the internal components of the cooling system to provide the corrosion protection. The corrosion protection takes a while to build up and breaks down relatively quickly.
Organic Acid Technology (OAT) - Newer type that relies on chemical corrosion inhibitors to actively prevent corrosion. It starts working as soon as it is used and last for up to five years. Usually this is pink, orange or red, but can also be blue or green! OAT is what XTs came with from the factory and is the recommended type to use.
Hybrid Organic Technology (HOAT) - Effectively a carefully concocted mix of both the above, but with borates instead of silicates. This is generally orange or yellow. It is nowhere near as common as the above two types.
The big problem is none of them are compatible with each other and don't mix without causing problems and any type can be pretty much any colour! For example, if you've had IAT coolant in a cooling system and you change to OAT, you'll get small weeps and leaks everywhere and eventually blocked radiators as the OAT chemicals react with the silicate coating left by the IAT, releasing it and producing a thick jelly-like substance. Just draining and refilling is not good enough as the silicates are adhered to the insides of the pipework and coating the waterways. A full flush (or two) is necessary. (OAT and HOAT don't mix either without causing strange reactions.)
OAT coolant attacks silicone too. So if you have fitted silicone hoses or have used RTV sealant anywhere in the cooling system it should not be used. OAT-safe silicone hoses can be found, but the vast majority out there are not.
Basically, if you don't know what's in your cooling system, don't go on colour, just assume it's not compatible with your new coolant and flush the system thoroughly before re-filling... which unfortunately means removing the tank and radiator!
