World War 2 war artists often used a sketchy, watercolor based style that had been fashionable in advertising illustration during the late 1930s. Nothing really wrong with that. Except many of those artists didn't depict military equipment convincingly, so the combination of free style and sloppy drawing makes such depictions useless to me and perhaps others who care about accuracy.
One war artist who did a decent job was New Zealander Peter Mcintyre (1910-1995). Biographical information on Mcintyre can be found here, though the writer unnecessarily lets his modernist bias show.
Mcintyre strikes me as having been a solid artist who incorporated modernist simplifications in some of his works, but did not usually take them very far. From a technical standpoint, in a number of instances his oil and watercolor paintings have a similar appearance, at least when seen on a computer screen. In his war work Mcintyre does best depicting people, falling down a little sometimes when dealing with airplanes and tanks.
Nice, strong self-portrait. Better yet, it seems quite accurate when compared to the photo that was probably taken later.
Another comparison just for the hell of it. Below is Nevinson's iconic take on French machine gunners. Mcintyre might have been aware of the Nevinson painting, but his version is pretty static and undramatic. Perhaps that's the way it really was when he passed by the team.
Mcintyre was a war artist for the New Zealand army which saw most of its action in Greece, North Africa and Italy during World War 2.
New Zealand commander.
The source for this image states that it was done in oil paints.
A New Zealand scene done in watercolor.