Chromatic Black

Chromatic black is a mixed paint color that looks black but doesn't contain any black pigment in it. None of the pigments in a chromatic black mix have a PBk (Pigment Black) Color Index. Instead, a chromatic black is created by mixing dark versions of other colors, typically a red and green or blue and red.

Why Use Chromatic Black?
Given how easy it is to squeeze paint out of a tube, why would you bother mixing up a substitute for black? It's partly the fault of the Impressionists (such as Renoir and Monet) and statements they made about shadows not being black and how it should never be used (although most of them did at some stage or other).

It's partly because using too much black to darken colors easily results in muddy colors. This is especially true amongst beginners, so some art tutors find it easiest to ban black altogether. It's partly because black can be a very flat and dull color. And it's partly because a chromatic black is a more complex, interesting color, with a subtlety that straight black lacks.