I recently posted some illustrations by the great watercolor artist John Gannam, who had an astute eye for color.
But before he became famous for his use of color, Gannam did hundreds of black and white spot illustrations in the 1930s and 40s for lesser publications.
Many artists treated such assignments as hack work to rush through. However, Gannam applied himself and learned valuable lessons from black and white pictures which served him well when he later painted color illustrations.
Specifically, he learned from black and white illustrations to master value, which refers to the darkness or lightness of a color. Working in black and white, Gannam taught himself to control the "value structure" of his pictures, balancing blacks and whites and grays. He mastered tonality and contrast and learned to make his pictures eminently readable.
I find Gannam's control over the value scale in pictures such as this one absolutely astounding:
If you don't think this is hard to do, try it sometime.
For years, Gannam refined his color skills by doing black and white pictures.
As Gannam's career advanced, color slowly crept into his illustrations:
When Gannam later graduated to full page color illustrations, he had great appreciation for the values of the colors he used. Much of the strength of his color paintings came from the lessons he learned during that long apprenticeship doing black and white pictures.