Yes, it's my impression too that only stereo copies were colored. Never seen blue or yellow, though! I think I've seen someone (possibly Chuck) state that the coloured vinyl was of inferior quality, so I've always avoided them, but some titles don't seem to turn up as stereo pressings on black vinyl too often. Mono copies seem to be a lot more common of many titles.
How do you think the sound is on the red ones, Peter?