Some martial arts like Karate and Taekwondo have different belt orders but here is the belt order for Jiu-jitsu:
White, Yellow, Orange, Green, Blue, Purple, Brown, Black, and Red
Red belt is usually reserved for a grandmaster of Jiu-jitsu, equivalent to a 9th degree Black belt. That should go to the admin of the forum/site (which would be Jen in this case)
I think 20 posts for whitebelt is good as you suggested. I think that the number of posts should go up progressively between each ranking, so I think the simplist way of doing just doubling the post requirement for each rank.
White = 20
Yellow = 80
Orange = 160
Green = 320
Blue = 640
Purple = 1600
Brown = 3200
Black = 6400
After black belt, you can have like 2nd, 3rd, 4th, degree blackbelt if necessary. I know the post count seems ridiculously high, but this way its more futureproof. I've seen some people at sherdog forums with 5000+ posts.
But as for what rank to call each color belt, thats a bit harder to do. Beginner - White belt is a good start though
