Here's a picture if anyone wants to know what it looks like: http://tinypic.com/r/2chmis5/5
Such an upper lip discoloration can be due to smoking, post inflammatory hypopigmentation, continuous lip licking or cheilitis. I suggest you to get an evaluation done from a dermatologist and get these possibilities ruled out.
I hope it helps. Take care and regards.
I have that exact same problem and it's a daily frustration for me. I've been to two dermatologists and neither had an answer. I've searched extensively online but found nothing. Did you ever find a solution for this?