Polygon to sub division problem
 

like you can see in the red box of the picture the area is darker then the other parts , i dont know why it is . all of it was modelled by boxes and converted to subdivision so there is no extra detail that make it looks like it does . and its on '3' smooth
Please HHElp

show a wireframe of the lowpoly object

Make sure all the normals are facing the all the same way, and check also that you dont have duplicated faces in that area.