If I get premium, and i buy a custom palette from the forums, when my premium runs out will I get to keep my palette? Or will it just kinda- dissapear/reset?

I'm not sure i can give you a straight-froward answer but my guess is that it will go away but i could be wrong.

Yes, once your premium membership expires, your palette will revert to the default basic palette and it won't be accessible until you renew your membership ^^