Should be writing but I did a search at the Van Gogh letters site, and found one mention of the English word, “colors“. Here’s an excerpt, emphasis mine:

Because instead of trying to render exactly what I have before my eyes, I use colour more arbitrarily in order to express myself forcefully. Well, let’s let that lie as far as theory goes, but I’m going to give you an example of what I mean.

I’d like to do the portrait of an artist friend who dreams great dreams, who works as the nightingale sings, because that’s his nature.

This man will be blond. I’d like to put in the painting my appreciation, my love that I have for him.

I’ll paint him, then, just as he is, as faithfully as I can — to begin with.
But the painting isn’t finished like that. To finish it, I’m now going to be an arbitrary colourist.

I exaggerate the blond of the hair, I come to orange tones, chromes, pale lemon. Behind the head — instead of painting the dull wall of the mean room, I paint the infinite.

I make a simple background of the richest, most intense blue that I can prepare, and with this simple combination, the brightly lit blond head, against this rich blue background achieves a mysterious effect, like a star in the deep azure.

Love that: “I paint the infinite.”