Hm, in that case the sequence above must be adjusted. The theory (untested by me, at least) is:
When you gain level X:
Bonus table index B is (X mod 15), which means the remainder of X divided by 15.
B | Options
-- ---------------
0 | Yellow / Purple
1 | Blue / Brown
2 | Green / Purple
3 | Red / Brown
4 | Blue / Green
5 | Red / Yellow
6 | Purple / Brown
7 | Green / Yellow
8 | Blue / Red
9 | Green / Brown
10 | Blue / Yellow
11 | Red / Purple
12 | Yellow / Brown
13 | Blue / Purple
14 | Green / Red