I'm suspecting a curved edge of some sort, where a shadow type coloring or depth of field type coloring is used. Spray stain/pigment from one direction, or such, to highlight deep areas or to highlight one part of the surface, other part(s) remain non-"colored". Result: deep areas dark colored, top areas lighter colored?
Similarly, like some faux finishing/painting? Paint an uneven surface and wipe the paint off the top surfaces, leaving paint on the lower surfaces... or different colored paint on each surface depth.