Drawing Shadows for Inlays on Leather Cover

For my current project, I have to draw shadows of objects inlaid on the book cover on the leather of the cover material.

I have worked with inlays a lot. I have messed with leather in different ways during the years of my bookbinding experience. However, I have never tried to paint leather in any ways or to draw something like shadows.

The general idea is to create recesses in the cardboard material of the cover, to accommodate for the printed photos. Right before gluing the photos to the cover, I should create these shadows somehow.

How would you achieve that effect on leather? What paint would you use?

Leather is crème in color. Maybe a bit warmer than crème. Therefore, I suppose the shadow shouldn’t be black or gray but something colder, like dark-dark-blue.

Any advice? =)