For example for a question named: What colour is shiny primal groudon? What sites are seen as reliable to get this information off? For example: Pokemon DB (of course) , bulbapedia and serebii? Or is it any?
It's left to the asker and the answerer. There aren't any rules for this, but here's an idea:
For the example, obviously an official site is best, but if they don't have the info, then look for it on Bulbapedia or Serebii since they're your next best options.