Firstly, two of the asteroids are almost the same shade of pink/purple which makes them extremely hard to distinguish other than shooting them and see how much damage they take.
Secondly, I'd like to suggest a more intuitive color scheme: follow the rainbow. So that would be red, orange, yellow, green, teal, blue and violet, in that order. Most people should be familiar with that sequence of colors, so it would make recognizing the tier of each asteroid easier without having to remember an arbitrary color...