Their are paints made for Bathrooms that handle the moist environment better. Also it would be best to stick with a gloss or semi gloss in a bathroom. Hardware or home store should easily help you pick one out that will work in a bath. Figure the area for coverage you will need so you can buy the right amount. No reason to buy a gallon if you are only going to use a quart. You may want to consider a primer/sealer to help block out that horrible green.