Ahh. No - this is not documented anywhere, but I can give a short explanation:
The spinners represent background worker threads. One spinner is one thread, so if you see 4 of these spinning, that means you have 4 build/test tasks running in the background.
The colours are all configurable in the global settings, but the defaults are:
Blue - Initialising
Green - All tests are passing
Red - Test failure (shown number provides number of failing tests)
Orange - Build failure (shown number provides number of projects that are failing to build)