In the heat (which I have plenty of here), my baseball caps change color (from "winter" to summer).  I generally wear darker colored hats in cooler weather.  In the past few weeks, I've switched to my summer head gear.  My ball caps are light colors (white to tan) and often have mesh panels on either side to aid in ventilation (like   (You need to Login or Register...I prefer ones with lighter-colored side mesh panels, but these seem to be more common).

I generally don't have to wet my cap ... it gets that way naturally.  But I do pull it off ever so often to give it a quick fan in the air ... that helps to cool the moisture already in my cap.  In the hottest part of the summer, I wear a rolled bandana above my brow (tied in the back) to stop the constant drip of perspiration down my face.

dd