What boon said. People will start faking admin messages with colors and whatnot. Plus I don't really feel that admins somehow deserve more respect (sounds like a power trip) than other players, and it would get annoying to have brightly colored messages from admins every time they spoke. All players should respect each other, admin or not. Admins just have the ability to take action when somebody is causing problems. Besides, many times admins want to remain low-key. People act differently when they know an admin is around. And you can use colors yourself, anyway, for example, at the console type:
say "<ctrl-K>Aplease reconnect!"