Hi Giulio,
Shaun is correct....In Creo 3, users can control the element colors using color tree.
http://learningexchange.ptc.com/tutorial/3471/color-tree-for-simulate-objects
By default Creo Simulate uses three different shades of blue for brick/wedge/tetra elements. For 2D quad/tri elements, two different shades of green are used.
This is to help distinguish the different type of elements used.
Thanks,
Mark Fischer