One easy and reliable way to mark is to use a simple technique common to laser cutting. Shallow cuts or engravings are covered with paint, then the excess wiped away.
What happens is that any paint that got into the shallow cuts stays there, resulting in a mark the same color as the paint.
Acrylic paints tend to work best. They are water-soluble (as long as it hasn't dried) and is a usually a bit on the goopy side which tends to help the process.
The good news is that even though acrylic paint won't stick to acetal (also called Delrin), this technique works great for two reasons:
- The paint in the marks is being physically held there, inside the shallow cuts. It doesn't depend on the paint "sticking" to the surface.
- Wiping away excess paint completely (the key to success with this method) is super easy to do. Because paint doesn't want to stick to Delrin, the excess wipes off no problem.