Thankfully the time difference makes it look like I made that comment only 1 day ago 
Hmmm.... I would probably need to spend a little more time searching to give you a more considered response (I feel like I'm at work ). The point being there are complicated options, but whether they are worth recommending, I'm not sure. If you are interested you can lookup some of the CSS that people came up with during the incarnation of the Smartly solution by searching for "smartly humidity color".
I may consider something along these lines myself, but no promises....
