ah... that's indeed a very nice concept :thumbup:

varying in size, shape and color (though the color doesn't matter here).
I think i have an idea:
You could add blocks with different weights, and every color would have it's own weight (blue = light, red=heavy etc).
This would make the game a lot harder and should only be used in the higher levels (or when the user selects a higher difficulty leve).
This way, you must put the heavy blocks at the bottom, so the tower will be more solid.

I dont know how hard it is to implement this. :? I guess you'll use the Newton library for this project. I dont know how it works but it might take Weight into account when performing the calculations.
You might also want to take the object's size/volume into account when specifying the weigth's. This way you could leave color as an unimportant factor and just make big objects heavier than the small ones.

I hope you like these idea's...

Good luck with this project

[offtopic]
A question to all native english speakers.. do i say "More heavy" or "heavier"
[/offtopic]