I'd start with motor oil and make it fairly strong, then I'd tone it down with brown. By tone it down, I mean use a toothpick dipped in the brown to bring the color down slowly. Both colors are available in X2. I'd get the shade of color you want then add the glitter.
Both of these colors can be mixed in at any point during the cooking. I'd add the motor oil to the raw plastic and cook it then add the brown. If a re-heat is needed during the color mixing, add a shot of stabilizer to the plastic and then work the brown in.