A few options here:
*Different coloured cut areas
**One way to do this is to paint one colour, mask it off (using a suitable material[Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable]), then paint a succeeding colour and repeat, planing or sanding off the excess paint[Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable] Wax helps[Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable]
**Another option is to cover the entire piece in adhesive vinyl, do the cutting, paint and then peel off the vinyl.[Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable]
**Or use epoxy: Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable — note that eyeshadow is supposed to be the sort of pigment used.[V-carve + resin inlay - #40 by jrsteurer]
Vendors:
Metals:
For marking engravings in metal, the traditional thing is a lacquer marker: Lacquer-Stik Paintstick — e.g., Amazon.com
Brass:
Patina solution — Traditional Black Patinas – Sculpt Nouveau [Engraving Brass? - #3 by mikep]
Alternatives
An alternative to engraving would be to use a drag knife to cut a resist mask and then chemically etch the material.[Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable]