I just finished performing this mod. It's was ridiculously easy. I used some 90% Isopropyl Alcohol to clean the insert first (allow the alcohol to dry fully), applied the sharpie to the insert (didn't really worry about using too much), used a fine brush to smooth out any rough spots, then used a dry (clean) cloth to gently rub away the excess on the ceramic portion of the insert. The hardest part was painting around the pearl in the triangle. End result looks awesome!
I didn't take a before shot, however, prior to this mod - while sitting in my watch box, I would look at the dull dark insert color and would be annoyed by how dark it was. Now, it looks like a proper 95% GEN looking insert!
@altesporsche you are a genius, thank you sir!