Could the answer be EYE ? Pronounced as one letter (I) Only contains two letters (E and Y). Two only in me (Most people have 2 eyes) Not sure about the double / single bit. Black, blue and grey are eye colours. Read from both ends and the same either way (the word EYE is a palindrome)
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, And Gray. Im Read From Both Ends, And The Same Either Way. What Am I?
The correct answer is eye.
To add to the above answer, you can have double vision or single vision. The rest of it makes sense. The rest of the answer I mean