When an artistic style becomes fashionable, wannabes swarm in. I'm not quite sure that I can truly label R. John Holmgren (1897-1963) a "wannabe" or "Chameleon" (as the title of this post has it). That's because there is little of Holmgren's work to be found on the Internet.
Yet Holmgren seems to have been a fairly well known illustrator in his day. Walt Reed in "The Illustrator in America, 1860-2000" notes: "His illustrations appeared in most of the national magazines and for many advertisers, including Chevrolet, Ford, Alcoa, White Rock and Cunard Lines. A long-time member of the Society of Illustrators, Holmgren was its president from 1941 to 1944."
The White Rock illustrations included the "Psyche" girl in various settings done in 1940s wash-style. But I want to focus here on the work he was producing in the late 1920s and into the mid-1930s. There were some illustrators in those days with strong styles that were popular with viewers. These included Norman Rockwell, J.C. Leyendecker, Mead Schaeffer, Henry Raleigh and Walter Biggs -- none of whose work could be confused with that of the others.
And then there were McClelland Barclay and John LaGatta, two other major illustrators back then. Barclay favored oil paints and used a form of Cloisonnism, outlining to emphasize his subjects. LaGatta usually drew images in charcoal and then applied oil washes to add color; he also favored outlining.
The point of this post is that Holmgren tended to mimic Barclay and, to a lesser extent, LaGatta in those days. I'm not sure what medium he painted in, though he seems to have made use of line and washes of some kind. Aside from seeming derivative, his illustrations were nicely done. Take a look: