ok well it's easy to solve... You can't really smoooth the top without smoothing the bottom. The problem is look how many verts you have on the bottom half and then look at all the verts on the top. That will cause a messed up mesh and it will discolour the bottom (this only happens in the preview not the render).

You need to use cuts and extrudes to keep the bottom faces hard whilst smoothing the top.

Alan