I know this feature isn't in yet (and that the shadows patreon goal is some distance away).
But.. As an alternate to blending directional light (as seen in that free-look 3d preview) smoothly, it'd be useful to set one colour out of the palette as "highlight", and another colour as "shadow", and overlay these colours at full opacity. To get some control over the effect, they'd need to have threshold gauges.
This makes it easy to adopt directional light on the 3d model for use on and in compliance with the hardware requirements of various pre-3d retro consoles such as the sega master system, nes/famicom, genesis/mega drive and so on.
I marked this as done as I have implemented shadows and lighting which don't introduce new colors. I dig your idea but I've decided to go with automatic lighting.