color of maps

Ok, so I'm making a sort of voxel isometric game where how it draws the cube is that it draw starting from the bottom to the top with:

for y = 0 to 8 screen.drawMap(map,0,0+y,124,124) end //just an example no errors are given with my code

And I was wondering how I could add shading to this? I've tried many different approaches with no success.