I was 3 minutes into testing the current trial version of Affinity Designer when I ran into the question of "Where is select by stroke and fill colours"?
I just needed to create basic colour variants of a logo. The way it was drawn makes Affinity software unusable for this tiny, routine task. I'm sorry to say this is so basic to any designer's needs it knocks you completely out of consideration for purchase at this point, or for recommendation.
I started this tiny test full of your glowing reviews, recent awards and reasonable pricing, expecting I'd likely end up buying the full suite on both Mac and iOS this summer. After reading this thread, that's not going to happen in this generation of your tools. Count me as one more voice saying that until you fix this you're not even in the game.