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The palace allows you to make contributions in any color.. It says, "However, for each box you may only use one color." Does this only apply when making multiple contributions in a round or in general? How would you keep track of the previous colors used if it is the latter?
It's a general rule, with nothing to track. If I am trying to fulfill the #5 box, then all of the cards I play must lead to the same color, whether it be through wild cards, card abilities, or unaltered colors. The same holds true for the other three boxes. So, for example, I could play 5 red and 4 blue and place cubes on the first two boxes.
It matters not what color you use per box, so long as all cards played for that box lead to the same color.
Hope that helps.