This was an interesting exercise. I had heard that Van Gogh was a regular correspondent with his brother, but the sheer volume of correspondence took me by surprise, well over 900 so far as I could see.
I found the following site the best repository of the letters
Reading through the letters gives a real sense of period and Van Gogh’s powers of observation are remarkable.
Attached is a sheet of a few extracts from letters and my comments and learning points from them.
Whilst researching this subject I also came across this fascinating account of how Van Gogh’s use of colour was influenced by the time he spent in Paris and his exposure to other artists there.
This was a good exercise, without it I would not have the same insight that I now have to Van Gogh’s approach to painting.