Really not needed... half the fun of transmog is finding bits that fit and match - if you could just colour your gear then nobody would bother farming those heroic plate boots from TBC every day for a month just because they're the right shade of purple... They'd just pick up some green ones on the AH and apply a dye.
And if you want a rainbow set there are enough off-set bits from AQ40 and BWL to satisfy even the most camp of rainbow warriors.