Why're my lips purple?

They aren't completely purple, but it happened a couple of weeks ago (I first noticed it after I ate some grapes which I don't usually have, so I just thought it was some staining). I often bite my lips so I thought it could be bruising, but would bruising last this long? They don't hurt when I press them either.

Any ideas? I don't feel particularly different (apart from the fact that I caught a cold two days ago). I'll answer any questions by e-mail if needed.