Extrude

I have CorelDraw 11 and make 3 Squares right next to one another. Then I color the middel one Yellow and the 2 end ones Brown. Then I Group all three of them.

Then I take all 3 Squares that I just Grouped and select it and go to my Extrude Tool Box.

And yes I can make it 3D and it works and the 3 Colors Extrude as well it all works.

Now say I start this whol thing again but now I make my 2 end Squares Bigger then the Middel one. Then I Group them and Extrude them it still works but the Yellow one is see through from above.

But like I say if I keep all 3 Squares the same size they all stay a Solid Color when I Extrude them.

Why I just wanted to know?