The multi-colored Smurfs appeared in The Egg and the Smurfs. They are a trio of ordinary Smurfs who use the magic egg to wish to be other colors instead of blue: one becomes yellow, one red and the other one becomes purple with green stripes.
When a Smurf wishes to the egg to be big, the purple-with-green-stripes Smurf dismisses him as not big enough compared to the elephant another Smurf had wished for, so the Smurf wishes to be even bigger until he ends up as a gargantuan Smurf.
When Papa Smurf undoes all the wishes, the former yellow Smurf and purple-with-green-stripes Smurf states that they want to get their weird colors back. Even though the formerly red Smurf doesn't say anything, it can be guessed that he would also want to be red again. Anyway, the egg breaks to reveal a little chicken before any Smurf can ask any wish back.
While the normal identities of the three Smurfs aren't stated in the story, the purple-with-green-stripes Smurf may be or not Vanity Smurf, due to the flower on his hat. Despite the fact that "personalized" Smurf costumes didn't exist at the time (save for Papa Smurf and Brainy Smurf), Vanity had used a flower on his hat from time to time (for example, in The Astro Smurf, where he uses a flower when Handy Smurf returns him a pot, but doesn't have it when he's welcoming Astro "back to Earth", and recalls the fight they had when he was disguised as a Swoof). It can also be argued that, just like the early Vanity, this Smurf put a flower on his hat for a moment and then eschewed it.