One simple approach (before getting into the photoshop/illustrator specifics) is just converting each icon one at a time to grayscale. Then we get a full ranges of grays and shades of let's say skin tons (👍🏻✌🏼🖖🏽) just become different grays. That's less than ideal.
Ideally what I'd like is to specify individual colors and color ranges and apply sort of a texture to it, like a stipple or cross-hatch or dithering texture perhaps, I don't know yet what might look decent. But say you could define a macro/action that acted on each icon, looked for the specific colors, and applied a specific line pattern or cross-hatching texture to it. Then you could make it purely blac and white perhaps.
But what I'm wondering for this question is how to get started on such an automation task in either Illustrator or Photoshop. I can get the 2000+ SVG emojis here, but then the question is how to go through that many and efficiently (programmatically) adjust them to black and white?
For the sake of coming up with an example, let's say that I wanted to have 5 shades of black: white, black, and 3 cross-hatching shades. I would go through and look at the colors in the Twitter emojis and -- say -- mark down "green" as looking good as a cross-hath 2 sample, and brown as cross-hatch 3, and green and brown next to each other as cross-hatch 1 by cross-hatch 3, etc. Stuff like that. Then I push "apply" and it converts all the icons to black and white. Is that possible? Something like that?
What is your recommended approach for converting all the SVG to black and white (ideally black and white, but if not b+w, then b+w+gray).
My google search for "convert flat icon to black and white cross hatch" doesn't return anything...