I think the statements about coloured LEDs being more efficient were in comparison to a non-LED white light source covered by a coloured filter - so it is more efficient to use red LEDs for vehicle brake lights because they only produce light in the desired band, whereas conventional lights produce a broader spectrum, most of which is thrown away as it passes through (actually doesn’t pass through) the red plastic lens.
No, I’ve been thinking about this, and it makes sense that white LEDs which use a UV emitter to energize phosphors would be more efficient than normal LEDs which do not need or use phosphors. There is some loss, since not all the UV radiation is converted to visible light by the phosphors.
Chronos: Chip designers have come up with efficient and reliable methods to drive LED arrays from 120 VAC. The Supertex HV9906 is one such example. It can drive hundreds of LEDs from 120 VAC, and doesn’t even require a transformer. It employs a nifty switching scheme that precisely controls current from the power mains. The data sheet is here.