Red lights often emit more heat and light than blue/purple lights, which some claim will disrupt the sleep of your bearded dragons. As they are not the full range of white light seen during the day, most beardies either learn to sleep through it or never seem to see it at all. A handful seem to be more sensitive to it, much like people - some people (myself included) can sleep just fine with red lights or blue lights on at night, but some people can't sleep with any light whatsoever.
Ceramic heat emitters are far and away the most efficient way to heat your beardie, lasting much longer and not emitting any visible light. They last so long that over the course of years, they will cost less than buying a red or blue bulb now, but initially they do cost considerably more than the colored glass bulbs.
-Jen