I apologise if I'm missing something obvious, I've searched through the Affinity Photo online manual and the help forum and not found anything.
Every graphics app I've used until Affinity has supported using multiple layers to constrain fills from the flood fill (paint bucket) tool - think of scanning in a line drawing, putting it on an upper layer, setting to multiply and using the line art to constrain fills on a layer or layers below. (I'm talking all pixel layers, BTW, no vector stuff).
The flood fill tool has a control for tolerance and toggle for non/contiguous but no control to respond to single or multiple layers. Is there another way of doing this or is the feature not yet present?