Tiny earthworms should be pink. As Nathan said, they look exactly like adult worms except small. If you have a ton of really white thread-like worms, they are probably whiteworms (Enchytraeus). I have seen these in my worm bin too, but they have disappeared recently, perhaps because the real worms are getting more crowded. They are harmless. Whiteworms make good food for baby axolotls, if you are raising any.