Cogito's answer was the right one. Cogito, master of answering riddles, would you care to give us a new one?
Sigh. I don't even really like riddles all that much. But okay. Pronounced as one letter and written in three. Two letters there are and two only in me. I'm double, I'm single. I'm black, blue, green and gray. I'm read from both sides and the same either way.