Humbrol 66) slightly thinned so it flows nicely over the paper.
Then individual slates are picked out is different colours. I think I used about 6 variations on the standard grey. Some were darker but most lighter. One is even more green than grey. The result looks too gaudy but I'll tone it down with a wash of grey later.
This looks like a worse job than it is. An old pointy brush, the sort that's long since lost its sharp point but is still got tightly grouped bristles, and slightly larger than scale slates help. I've tried to do this will lesser brushes and keeping the paint on a single slate is impossible and frustration is inevitable.