This system was nice when there was 1 or 2 colors by banner and so 21 different possibilities. Even, if some of us waited for the Yellow/Brown banner, it was reasonable.
Now we can have 3 colors by banner… It means a lot of different banners like these ones: +2 Purple, +1 Green, -1 Red or +2 Green, +1 Purple, -1 Red or +2 Purple, +1 Red, -1 Green. There are… 120 possibilities (6x5x4).
Do we really have to wait 20 years to have all these different possibilities? 
So a solution is to choose 3 different colors instead of 1 banner:
- +2 color bonus
- +1 color bonus
- -1 color malus
If the 3 colors are different, an actual 3-colors banner.
If same color for 2) & 3), a mono-color banner.
If same color for 1) & 3), a 2-colors banner.
If same color for 1) & 2), a new kind of 2-colors banner that provides +3 bonus to one color… Maybe this kind of banners should not be allowed.
One can imagine that you can create your own banner with 3 different parts with each parts have 6 possibilities. The interface could be the same than the one for the Hero.
And yes, I want my +2 Purple, +1 Brown, -1 Red banner ;-).