Could you have multi-color symbols that combine with both colors as usual, but where that kind of multi-color symbol only activates itself if other symbols with both colors are present in some combination? To encourage multi-color decks?
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That's an interesting idea. I wouldn't want too many of them due to complexity creep, but something like Diamond but off color (like, a Red symbol that deals damage depending on how many Green you have) or something that scales with the number of different colors in your deck would at the very least provide more support for rainbow decks (which are currently only held by the actual Rainbow symbol)