They will provide a letter grade at your request. They don't guarantee it but it looks like you can get an A with a combination of green and blue. Although they mentioned contribution in class and on the forum as part of the determination, I imagine that you should get an A even without contributions if you have all blue in all four categories every week. As for an A+, I am not sure if just all blue will do it.
There is a huge range for blue. As far as I can tell, it is not based on a pure percentage but takes into account the difficulty levels of the problems that week. I do know that it is possible (at least sometimes) to reach blue after completing only 6 of 9 challenge problems if you get it correct on the first try. Similarly, I know that you can still get a blue on a proof with only 6.5 out of 8 possible points. We only have experience with Algebra 2 and Geometry so not sure if it is true across the board.
I believe that grading is "easy" because the material is supposed to be hard and AoPS wants to discourage the 100% mindset.
