Its not clean, but if you do "rules text:" select either include or exclude, and then the color coes you want/don't want to see in rules text, it'll filter as such.
For example, once I've narrowed a list to just commander/brawl/singleton legal within the colors i want, then I go and (ina second tab) copy that search. In second tab, add the "rules text" to filter cards that include colors I don't want to see (there will be a few false positives, but not many). From this, adding the following filter to first tab "name - does not contain:" and listing the ones I cannot add (again, if you've narrowed tab 2 proparly, it should not be a long list), then apply filters. Finally I save the search (just in case I need to return to it) and begin the hunt.
Is it elegant, no, but it does work.