It is much harder than it looks and I don't know how he managed to mix so many different yellows, as I struggled somewhat. Yellow is made chemically nowadays unless you spend a large sum on pure cadmium paint. I did so - for one tube only - and must admit that my new pure cadmium yellow is more luminous, a deeper colour and easier to use than the less expensive version. Doesn't mean the painting is any better of course.
Van Gogh planned a series to decorate his Yellow House in Arles as he explained to his brother in this extract from his letter of August 1888
My Dear Theo
I am hard at it, painting with the enthusiasm of a Marseillais eating bouillabaisse, which won't surprise you when you know that what I'm at is the painting of some big sunflowers.
I have three canvases going - 1st, 3 huge flowers in a green vase with a light background; 2nd 3 flowers, 1 gone to seed, having lost its petals…3rd, 12 flowers and buds in a yellow vase. The last one is therefore light on light, and I hope it will be the best. Probably I shall not stop at that. Now that I hope to live with Gauguin in a studio of our own, I want to make decorations for the studio…If I carry out this idea there will be a dozen panels. So the whole thing will be a symphony in blue and yellow. I am working at it every morning from sunrise on, for the flowers fade so soon, and the thing is to do the whole in one rush. If I carry out this idea there will be a dozen panels.
You were quite right to tell Tasset that he must give us some tubes of colour..when I have finished these sunflowers, I may need blue and yellow perhaps…
If you want you can read all or any of Van Gogh's letters free on line here . This painting is the 3rd described in his letter above.