I am trying to make something inspired by the look of Ludwig Hohlwein’s posters.
It appears his work primarily involved watercolor painting with different drying times in order to achieve these characteristic modulations of shading.
What techniques could I use to imitate this style (in particular, the flat-colored shadings) starting with a photograph?
I am interested in the image manipulation process (preferably using Photoshop and/or Illustrator) as well as the choice of a suited photograph (e.g. the lighting).
Although there is no substitute for a real watercolour illustration, it is possible to get a similar look, or as you say, à la Ludwig Hohlwein.
Clearly the choice of photograph will play it’s part. Something with defined shadows and highlights, costume from that era, a background which can be easily removed, etc.
Here I used a combination of an image with the background removed, to which I applied Photoshop’s Cutout filter, then I found a textured background, clipped the artwork to the texture layer, and set the layer blending mode to Colour Burn. You’ll need to click on the image below to see it in detail.
I also tweaked the image a bit by adding a hue-stauration adjustment layer. Here you can see the arrangement of all the layers.