[What exactly am I doing...and can this be replicated in photoshop/other programs?]

Hi everyone, so I use Affinity Photo as my "photoshop replacement", and I usually make art in it.

Well I was messing around in Affinity Photo, and when I dragged a pixel layer over to another pixel layer that had my line art on it, it allowed me to change the "color" my line art by painting over it like a mask, as opposed to it being just solid black.

(see image 1& 2, versus image 3)

This is a neat little feature, but I don't know exactly what it is, or how I am even doing it.

You can see in image 04 how I accomplish this layer trick by dragging it over and it shows this option (see image 04).

Any help would be appreciated!!