The technique is called dry brushing - and on something that’s already three dimensional it’s shockingly easy - and as a bonus, if you don’t like what you did you can always laser another one! This video has a nice overview. I tend to use a much smaller brush (1/4" wide or so), and dump less paint on the paper towel, but this is pretty similar
I used a mix of white/black/grey - and didn’t mix them smoothly which got a random mix of those colours. The other option is to do each colour individually starting with the darkest.