It might be the normals, or just how the vertices in that area are sitting at the moment. Check like KB said the normals are all facing the right way and to see if all the border edges are merged. You can also go edit polygons go down to normals, in the little window that pops up go to soften/harden option box, click on all soft and than apply. This should align the vertices that might be causing the color changes to give you a flater look.
I am enough of an artist to draw freely upon my imagination, knowledge is limited, imagination encircles the world. (Albert Einstein)