Mark Rothko. No. 16 (Red, Brown, and Black). 1958
The artist's son, Christopher Rothko: He would often start with pen-and-ink sketches, working out proportions. And then he would paint a background color and probably paint several over that in a very thinned-out stain of paint so that these different colors that are layered one over the other will shine through.
Then he would paint rectangles over that. And oftentimes these paintings would be painted relatively quickly or at least stages of them would be painted quickly but then he would stop and look at the painting for a very long time before proceeding with the next step, or before deciding that it was done.
Typically, in the 1950s, the canvasses were propped up on blocks. Near the floor and he would be up on ladders. Sometimes he would actually turn the painting upside down so as to be able to paint the top section more easily, which, of course, makes us all crazy when we try to figure out which way is up and the drip marks are going both directions.
He would actually have pulley systems to raise and lower the paintings, to help him not just paint it but also to try and get a sense of heights and proportions about where something would go in the room how it would work on the wall.
Later in his career, he did have some assistants but they would help him mix some paints. They would help him stretch some canvases. But then they would leave. He always painted by himself.