Stubbs' palette is typical of the period using Prussian Blue for the jockey's jacket and the sky, as well as in some mixtures for the landscape greens. Under magnification, occasional large 'lumps' of this pigment can be seen bleeding into the surrounding paint
Prussian blue on jacket (20x magnification)
In the mixed greens, a gritty yellow is easily visible, possibly Naples or ochre, with a bone or charcoal black and smaller amounts of lead white and Prussian blue. The sky has faded, as seen around the rebate edge, and would originally have been a stronger blue. It is quite possible that the jockey's jacket has also faded.
Yellow highlight at foreground (20x magnification)