Shinies are very rare versions of every thing and you have the possibilities of 1 out of 10,000. But whenever you rebirth, the chances of you getting a shiny increases (If you have a x10 multiplier then it would be 1 out of 1000). When you sell a shiny, it sells for a lot more money (10x more) instead of a normal producter / generator. Here's a list of everything with its shiny version.
Producers: