I have heard that the GBC can do more than 4 colo(u)rs per tile, has a secret high resolution mode, and can do 3D graphics in hardware. Is this true ?
None of this is true except possibly the first one. You might can do more than 4 colo(u)rs but only through creative palette manipulation during H-Blank. This is not practical for many things and is only achievable if you know exactly what you are doing. Read the GBC Specification in the Web Links Section for what video modes the GBC supports.