If you going to change the color in Layout you would go to Scene > Effects > Backdrop > and then select your color in the three boxes.
Well from your first object i can see that everything is the same color but on the second pic there is a part at the bottom that looks lighter than the rest of the head. If you were going to make it disappear then add a 100% transparency to it. Or if you want to include it with the rest of the head then you make a whole new surface including everything. Hopefully that doesn't throw you off
