Dover, a company based in Mineola, N.Y., published "Antique Automobiles Coloring Book," its first book for adults and more experienced artists, in 1970.

The trend has spawned coloring contests, a new National Coloring Book Day (Aug. 2 in the U.S.) and social-media pages for sharing completed pictures.

Given the popularity of adult coloring books, it’s only natural that other companies are trying their hand, too, including Brazos Bookstore, in Houston, which published 500 copies of its second ...