This seems unnecessary complicated in all solutions, and still not fixed in Jan 2024. If someone has a good solution, I would be very happy hearing about it. I think the OP is misunderstood, and many others in the thread. We are not looking for HSL adjustment, overlaying, selecting different parts of the photo. We would like to tell Affinity, that wherever the colour F8F2E4 pops up, it shall be replaced with 86BC30 for instance.
I am working on a simple webpage with 4-5 colours right now (not my area, I am using Affinity rarely as a hobby for personal photos, and I don't design webpages) for a friend, who asked me about colour palette ideas, and how we could check how the page would look like using them. Currently the main background colours are shades of grey, and the colour replacement brush tool doesn't do anything about them. Again, as this is a screenshot of a webpage, there is quite a lot of text everywhere, and I would like to avoid having to click into every nook and cranny, to select the areas I would like to work on.
The closest solution seems to be this, but it seems like you still have to select each object, and it is in Affinity Designer, not in Affinity Photo: