You cannot use color to assign meaning to something, as that poses an accessibility problem for those who are (color)blind.

I don't know if the tag feature has any accessibility for those who are blind. For colorblindness, we can try to use colors that are distinguishable by most color-blind people, but I get your point and think that color should not be our primary choice.

You've identified an issue we talked about during the design phase @Huji. The idea of clarifying the various search relationships in effect is an attractive one, and your idea of using color is quite interesting. Question: were you thinking that each possible filter group would be permanently assigned a different color? Or that a more limited set of colors would be assigned n the fly, since users are unlikely to use more than a few different groups at one time?

During testing, user making filter selections generally understood their choices (though few could have analyzed them, as you have done). So we have no mandate at this time to dive back in to make what would be a fairly complicated change to an already complicated interface. But we'll save this ticket for a future situation in which this interface is up for review.