You can watch me describe the techniques for this drawing style here in a talk I gave at the North American Cartographic Information Society in October 2019.
Last year I shared my technique for drawing hillshade with graphite (pencil). Since then, I have explored a few different methods for drawing color hillshade. I just posted one of those techniques that I've been exploring on my new website. This method uses those graphite drawings as the base, or the starting point. So if you'd like to get started on color hillshade, you can check out the graphite process here. You can see a map completed in this method at the bottom of this project post. Just scroll down!
I presented this technique at the annual North American Cartographic Information Society conference (NACIS) last week, and am also posting some images from that presentation as well.