I don't think a mono deck should be the test because 99% of mono decks are not viable after you graduate past AI-3. I would say the test would be: when constructing a rainbow deck, which element(s) is not represented?
By the rainbow criterion, I would pick Light or Water as the weakest element. Improved Miracle can be a lifesaver, but it competes with Sundials for the Light Quanta. Permafrost is nice against immortal creatures, but it doesn't have the all-around usefulness of Bone Wall and Phase Shield. When I know that I'll be facing a poison-heavy FG, I add a Purify or two, but not many FGs do use poison. So my basic rainbow has no Light or Water cards, but I do switch in the ones I mentioned when appropriate according to an Oracle prediction.