[Paintbrush colour frustration]

Hello - novice question, sorry:

How come I can't paint a white colour on this circle? It just won't show up - even if I create a new layer (but a dark colour will). I can see the white appearing in the layers panel - why is this not on top of the image, as the layers are seemingly at the top? Thanks

[Brush work within a shape]

Hello - this is embarrassingly simple, but I'm new to affinity and have spent hours trawling the net to find an example of how to do it.

I have a circle on the page and I want to manually colour it in with a brush - but I want to separate it from the background so that when I use the brush tool on the circle, the colour does not go outside of the edges of the circle. What's the approach for this? Does it need a layer mask?

Big thanks!