Just because you've seen 'em and the cars have been issued an MoT certificate does NOT mean that they're automatically legal on the road.
Blue coloured filament lamps do make cars look very silly, especially when the headlights are turned off. There's that blue reflection from the headlight interior.
Some lamps have a very subtle blue/purple filter - these to look quite good - and because it's so subtle there's no attenuation of light.
I've got rainbow sidelights in the Primera and I know these are not road legal as it says so on the packaging but it still passed it's mot so I do agree with you there HDi. I know the mechanic who MOT's my car an he is quite a good friend. Many people may have this same friend in the MOT business...