PLEASE , remove the grass cloth first. You will be happier with the final results. Grass is one of the easiest papers to remove, here is a link. Also if you are looking to change colors, it is very easy to just paint the grass, it is done all the time. An oil based primer should be applied first or if it is really stuck, as in applied without priming first, you could probably use Gardz as your primer and avoid the oil.