It actually began in the 80's, when Rainbow Brite merchandise would show up in tv shows.
We see an 18" Rainbow Brite doll in Miami Vice's Season 1 episode, "The Home Invaders:"
And I distinctly remember an episode of Who's The Boss where Mona is drinking tea or coffee out of a Rainbow Brite mug. But that image doesn't exist on the internet - yet.
And while she seems to have taken a break from the small screen in the 90's, she has shown up multiple times since the year 2000.
A Red Butler doll appeared in the Reba episode "Core Focus:"
A Rainbow Brite cosplayer showed up in Bones episode "The Princess and the Pear:"
Rainbow Brite bed sheets appeared on the HBO series "Girls:"
A Rainbow Brite doll was animated into the Family Guy episode "Prick Up Your Ears:"
She was parodied on The Soup:
And she's shown up on Robot Chicken five times.
Wikipedia mentions Rainbow Brite being referenced in songs, showing up in music videos, and being drawn into web comics. And check out RainbowBrite.co.uk's 'Rainbow Brite in Hollywood' Facebook album for a few more screen caps. I wonder where she'll pop up next! :)