Brain Teaser: Riddle me this

Can you solve this riddle? Post your answer in the comments section. I'll update this entry with the solution tomorrow.

Pronounced as one letter,
And written with three,
Two letters there are,
And two only in me.
I'm double, I'm single,
I'm black, blue, and gray,
I'm read from both ends,
And the same either way.
What am I?

Update: Yes!! "Eye" is the correct answer. Thanks for playing! :)