 Ultramarine Blue is such a lovely rich choice; a dark but  transparent colour that I find it hard to resist using it in a painting. It makes blacks intense and enlivens the edges between people and their background in the picture. 
I laid down a layer of the ultra blue over the grey of the sky, to enrich and vitalize it, then removed a lot of it with a large soft rag about the moon, giving the light a chance to shine intensely from there.Â
