My suggestion is a new layer property called ‘fill’. It would apply a color on all pixels in the layer while preserving alpha.
Potentially, you could add more options for how the color is mixed with the layer but for me the most useful would just be a straight up replacement of each pixel’s RGB. It’s essentially a ‘post effect’ on the layer, so disabling it would show the original pixels again.
When I animate, I like to use separate layers for each limb’s silhouette. I give each of them different colors so it’s easy to see how the animation flows for each limb.
A ‘fill’ property would allow me to set a layer’s color and keep my front and back colors to black and transparent so I don’t have to pick the specific color each time I swap layer. I swap between the limbs a lot and copy between frames and layers. Not having to recolor things by hand would greatly improve my workflow.
Thanks for a great piece of software!