Snowy night scenes, especially of Central London, have a wonderful appeal.
For this particular scene, I made a sketch of the street lamps, the trees, distant buildings, bench and railings.
I then used a palette that I had used earlier in the day, when I was teaching beginners how to mix skin tones. A dirty used palette is a wonderful source of a chromatic colour. Mix all the colours together. Unless of course there is a strong bias of one colour on your palette, you will have a colour that is usually perfect as a chromatic black. I had been using process magenta gouache, as well as all the other colours necessary for different skin tones, so you can see a strong bias of purple in my mix, which I was quite happy to use. I painted a wash of the colour on to dry paper, down to the horizon line. When that dried I applied a stronger coat of the mix (less water added). I then started painting in the lamp posts. I do not use masking fluid but if you do you could mask out the shapes of the lights. I prefer to use gouache over watercolour.
I then painted in the railings, and the tree trunks and branches. I applied a weak wash of my mix to the pavement area, using a stronger mix for the cast shadows. I painted in the bench with a mix of my purple and burnt sienna. I then painted in the distant buildings with the purple mix. I painted in the lights using a gouache white, which I then covered in part with a lemon yellow. The two mixed together slightly, which was the effect I wanted to achieve.
I used a round headed brush to stipple marks in the snow on the pavement. I then used a white gouache to add the snow to the branches of the tree. I then took some time to refine parts of the painting, putting more detail on the street lamps, the bench and the buildings in the background.
Using complimentary colours gives a good artistic effect, the yellow in a purple night scene is a perfect partner its compliment.