Riddle for 1/13-1/19

Last week's answer was:

"An onion."

This week's riddle is :

"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?