The electronics in the base are going bad. Usually after they start glowing pink, not soon after they decide to go out. Either with a puff of smoke, or a flame shooting out at one side of the glass coming out of the base.
It only really does it when it's cold out. The attic is above and makes the light in the exhaust fan cold. I have a cfl outside and it's just dim on start up when it's cold. The one in the bathroom is a dim pink when it's cold. Once it's warmed up, it's a bright white. I read somewhere that is was a lack of free mercury and that this should go away with use but it just now started doing this