If anyone was wondering why Worgon (the offspring of a Worgen Druid and Onyxia) has rainbows on her arms/legs/tail, it's because she had an unfortunate accident when she was little, she wasn't born with them.

Her father (the Worgen Druid) was an experimental alchemist and once he created an item that was part quartz stone and infused with magical elements and herbs and stuff, which ended up being a rainbow-coloured magical quartz stone, which when Worgon was a toddler she grabbed off of his table and bit into it, releasing the magical energy which then fused with her permanently.

And that's why Worgon has rainbows :)