Green is good for any healer except a disc priest, as it doesn't proc off of their spells. Green is technically better than red for a holy paladin, because red will result in significantly more overhealing than green, as it will simply double your heals. If you use divine light, for example, and it heals for 60K, most of that is usually going to be wasted, however if it instead heals the entire raid for an equal share of that heal, it is less likely to be wasted. Generally with anyone but a disc priest I get red and then blue, as green is better than red for most classes, and I would only have it for a short time before getting blue.
Of course blue is the best if you are 3 healing, as you will get twice your heals through 100% haste (2x as many heals per second), it lets you spam holy radiance with the reduced mana cost, which means more healing again, and then it adds more ticks to holy radiance, MORE HEALING! Just make sure you are casting holy radiance on a different target every time so that you don't overwrite the heal, which will destroy your healing.