This is the Study for 'La Grande Jatte' by Georges Seurat. Painted in 1884-5, it's an oil study done on a small board of unprimed wood. You can see where the wood surface shows through in some areas, like the water.
Seurat likely painted this on location, and as a morning sketch, the sunlight and shadows are reversed from what you'd see in the final painting, which depicts the scene in the afternoon. The sun is shining from the south-east, lighting up the foreground.
Despite all these shaded areas, Seurat uses touches of golden-yellow throughout to create a warm tone and unify the composition. His focus here isn't on the figures, but on the colours of the grassy area that fills almost three-quarters of the picture. You can see how he's used green tones to dominate the scene, with added strokes of orange, pink, wine red, and light blue for added depth and interest.
Seurat's colour choices were based on his direct observation of the landscape, but also informed by his knowledge of contemporary colour theory. He was experimenting with new ways of depicting light and shadow using colour, and this study shows some of that experimentation.
Study for 'La Grande Jatte' hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.