I know this is a necrobump, but I'm so glad I found this thread. I'm currently designing a card game with 6 "colors", and was having a headache on how to get a 3 color set to work. A 4 color set would allow for more equal representation of each color, but would be a headache to justify and build around. The idea of abusing hybrid mana and activated ability costs makes so much sense, I don't know why I didn't think of it before. I only have one faction fleshed out flavor-wise, and the set was already built around the idea of each faction being composed of two pairs in conflict. I think this would be the perfect step up to a 4 color design and I'm really curious to know what ideas people have come up with since this thread.