You should learn to do it without an indicator first. It keeps you more involved and you'll catch more fish because some are indicator shy. When I use one (usually) I like to use a white indicator as it matches bubbles and foam on the surface. The longer you can cast the more fish you can catch, you have to consider that it takes your nymph rig a while to get down to where the fish are at, many people fail to realize that. The longer your nymphs are on the bottom, the more fish you catch. LEAD THE FISH! Lead them by the greatest distance you can and get a drag free drift through where you think or know they're at.