This is watercolor too. I did this last week very, very fast. It's actually mostly acrylic on the figure over the watercolor. I've found that you need a hair dryer for the watercolor, or you end up with a mess really fast, and I got impatient with this one, so acrylic took over. I figure I'd like to find a balance between the two eventually.
Color mixing went very fast for this too. I'm getting more confident with it. My blues are greyed with orange, and my reds with green. I'm finding that my blues go toward orange too much, and I think I'm going to start using black and white to grey my colors a bit more along with compliments. It really depends on the color though. Because every color has a cast to it, and isn't a perfect blue or orange, when you mix them to get grey you can get unpredictable results, and I like that, but I want to get more control over it in a more predictable way, so straight up grey should help.