I created a 3D structure with three layers in Adobe Illustrator using the Extrude & Bevel effect. It consists of three slabs (orange and transparent, then grey and opaque and blue and transparent), where the grey one has holes inside. The orange slab should continue into the holes in the grey slab. As can be seen in the picture at the bottom, there are two separate orange blocks that have an interface together. This interface looks darker than it should, since two boundaries meet.
How can I remove or merge this interface (to make it look as if this is one solid block with cylinders sticking out from it)?