In Photoshop, you can set a simple preference for the eyedropper, to pick the color from the currently selected layer, or the resulting color from the displayed (flattened) set of all layers, or some other options:
I regularly paint on a layer underneath other layers, and in PS I can select colors from the layer I'm painting on while I work. In Affinity, when I use the color picker while I'm painting, it picks the resulting color from all layers, which means that new color is dramatically wrong when I apply it to the "underpainting" layer.
Is this an existing feature I haven't been able to find? Or is this simply not an option in Affinity?