It sounds like you might be thinking of the "Ponyville Factory" or the "Buttercup Factory", which are both featured in the My Little Pony: Friendship is Magic series.
* Ponyville Factory is a factory that produces various products, including toys and other items, that are used in Ponyville. It's not explicitly shown to be harmful or negative.
* Buttercup Factory is a factory that produces the delicious treats called "Buttercups" and other sugary snacks. While the factory has a somewhat ominous reputation, it's mostly portrayed as a place of fun and deliciousness.
Perhaps you're thinking of a fan-made concept, as "Rainbow Factory" sounds like it might be a term used in some fan communities.
Can you tell me more about what you're thinking of? Maybe I can help you find the information you're looking for!