They are also smart enough not too stand where they will hinder the player's or umpire's vision.
I'm not going to wate the time, but I can't recall too many games this past season with base umpires in the cremes. I watched plenty of games and went to a few different parks but never actually saw them in person. Thanks to ESPN, I know they exist but they seem to appear infrequently.
If you want to wear creme and your organization permits it, have a go at it. Mine doesn't and I haven't melted yet.
Those same guys that can't do the bases in a light blue shirt will wear a navy blue golf shirt for 18 holes in 90 degree weather though!