I've heard it is really difficult to get the "beam" or "ripple" effect from PC
lights. Mostly because the light source is over such a large area. If you want beams or ripple you need to go with a single-point light source, such as Metal Halide. You can put two over your tank, one over each half, and you'll get a nice ripple effect cast onto your tank bottom. The beams your looking for, however, only appear with stuff suspended in the water column. (like dust in the air) so those may prove to be rather elusive. If you consider the MH
approach, keep heat in mind. The bulbs can be VERY hot, but it will be like having the sun sitting over your tank.
My only concern on painting a shadow would be how the colors would affect the over all lighting in the tank. Do you think it would end up just tinting your tank green?