Restrictions:
If you look at it... it might not be a shader. For me it mitght be simply lowered resolution? Nevertheless, it has colors other than the two that are allowed. Since it has ones between the two. (And it's pretty unlikely to run on the Nokia 1130 but this doesn't really matter for me.)
Other:
Since it has such a low resolution, it is really hard to distinguish what's what. I think it would have been better to just give the ball a dark outline and leave it bright, since then it would be way easier to know where it is. It could have been a great game, but since it's so hard to see, it makes it so much less enjoyable. Not a bad game, but it would need some changes.