I think you'll find it's largely a photographic artifact concerning when and how you shoot them. At night they almost always look blue or blue-pink, but it's not very luminous and generally invisible in daylight. Also they tend to be blue when they're just starting up and the fuel/air mix is very lean. Video cameras tend to see it whiter than stills cameras since they tend to have less IR filtering on the sensors.
Must admit, I'm never quite sure why it's useful to be throwing all that flame out the back, since if it's burning outside the engine it surely can't be all that propulsively useful.
Phil