When they do the weather, they're not actually standing in front of a weather map, but instead a green screen onto which the station projects the maps (they know where to point by looking at a monitor, which gave us the Pete Bouchard Incident). So if they wore green, they'd look like they have parts missing (whatever is covered by the green). Matt Noyes reports:

A few years ago, got a complaint I wasn't wearing green on St. Paddy's Day. So, donned a green cloak & went invisible.