If you use a Grey Jar, does it give you back your Rainbow Jar? Or does it give you the choice to change it to another skin? Or do you just loose it forever and you've wasted 10 Gold Coins and 1,000s? 

Grey Jar only reverts the skin to the default official artwork, it doesn't do anything else.

Rainbow Jar is a single use item. There are no ways to reuse it.

Ok, thanks :) I was just wondering if it did, cos then I'd change the skin of one of my pets, thanks!