I learnt this the hard way when I went straight for it on the first attempt and ended up having to sand down and re-gesso the entire canvas because I was unhappy with what I had produced.
This sketch is still not quite right. So I had another go - smaller and using gouache - experimenting with different colour temperatures
These four occupy an A4 sheet of heavyweight watercolour paper. My thinking so far is that top left is too cool. This is supposed to be a hot day, not wintry. Bottom left is too night-time-feeling - this painting is supposed to be "into the light". Top right is just too much. Too orange? Anyway, the horse doesn't pop out, which he is supposed to. Bottom right I like. But reproducing that on a large scale and in oil paint without the result looking like a brush-wiping station is going to be a challenge.
Have a lovely weekend and thank you for looking at my paintings.