Don't wash them until you are ready to use them. If you want to go beyond red, you can find sea-green daikon radishes. Daikons are long with a white interior and can range from mild to very spicy. You can peel and slice daikon very thin and toss it in salads, or cube it or cut it into sticks to munch on. Daikon is great to cook with, tossing it into a stir-fry, rice and pasta.
Black radishes are a little harder to find.
They have a coal-black, rough skin with a blazingly white interior. Black radishes are almost always very spicy. They need to be soaked and then cooked to cut down the heat. Daikon and black radishes will keep in the refrigerator for several weeks.