That's definitely the most straightforward way to implement it, but I think the difference rule has more potential for neat puzzles - for example, you could give the player 1 Navy Key at the beginning of the level to essentially make the Blue aura require 4 Blue Keys instead of 3. I updated it again - my ruling on this is "the difference must be at least 1/3/5, and the amount of whichever one you have more of must be positive" - so if you have 2 Green Keys and -3 Forest Keys then you have the Green aura, but you cannot have the Green aura at all if you don't have at least 1 Green Key.