Do you want to know the exact codes for each user rank colour or just the general colour scheme for all user ranks?
It generally goes from blue to green to orange, red and purple (similar to the colours we used for our Japanese girl group)
You can climb up the ranks by being active on the community, it starts with logging in and hanging out, starting topics, replying, accepting solutions, giving and receiving kudos etc etc. There's quite a lot of stuff you can do in order to climb up the ranks, I can't tell you the secret for it, though
Bribing and complimenting the community manager or any other member of the crew won't help you with that. Just saying