Related question - what would you do if you wanted to add some color to these, start over fresh with a different technique, use this as an overlay to a color layer, or what?
Well, I'm still figuring these techniques out myself, but I imagine I would apply the color to a separate layer. Layers are great for keeping the various components separate and retaining the ability to adjust them relative to each other. Components include value (light and dark), color, outlines and highlights, among others. I only used two in the last set of sketches: value and outlines. Technically, I did some highlights with pure white but I combined them with the value layer. There are reasons, especially if you're using color, to do the highlights as a separate layer.