Why is this face model a different colour at the lips?

I am pretty new at modelling, its probs a stupid question but whey

You have inconsistant face normals. Some are pointing inwards and some are pointing outwards and you are trying to smooth shade between them.

Select all vertices and remove doubles (W / remove doubles) then recalculate normals (Ctrl+N). You can manully flip selected faces with W / Flip Normals