Value separations have been around for a long time. I was first aware of them when I was taking pictures for the school newspaper and annual at Clemson in the late 1960s. Here's one that was in the 1967 Clemson annual. (Not my photo.)
We considered this pretty far out at the time. The printing company did it from a straight black-and-white photo; so, to us photographers, it was kind of a magic process.
Later, at the University of Georgia in the mid 1970s, we learned how to do value separations for ourselves. We started out simply, using only photographic paper in the darkroom. Here are a couple of examples from that time:
We weren't allowed to stop after doing a "normal" looking separation; we had to do variations that put each of the three gray levels in each of the separation zones. Just to show that we could, you know. We also did value separations using Color Key material. These let you put a color or a texture in each separation zone. Here's a portrait of my photography professor:
I would call this portrait and the football image above "false color" separations, because the separation zones contain colors that aren't derived from the photos' original colors. (Although they do adhere to the originals' light/dark values.)
I like value separations. They remind me of some of the ink-based print processes like screen printing and lithography. So, being a programmer, I decided to write a program that would let me convert photos into value separations with some degree of control over the zones and values. (Although they are all "natural color" separations, because I'm currently too lazy to program a color picker.)
Here's the program applied to a grayscale original. It contains five separation zones, each with a different shade of gray.
Here's a similar treatment applied to a color image. In this case, the hues are all retained, but the colors in each separation zone are all the same brightness.
PS--After writing that my value separation program didn't have "false color" capability, I felt a little embarrassed. So I spent the afternoon upgrading it to include false color. Here's a not-so-special example, just to show what one looks like. There are five separation zones, each containing a single color.