I used the ColorMix shading app to add red shading to the original model of Darth Vader bust by Eastman.

The red "light" on the model is achieved by printing the highlighted areas in red filament, and the midtones are a mix of black and red. This is all done automatically by the app.

Learn more about Prusa ColorMix in the blog article.
To correctly open the 3MF project file (which includes virtual extruders), make sure to use PrusaSlicer 2.9.6 beta (or newer), which introduced support for color mixing. In older PrusaSlicer versions, the model would instead only use normal extruders.
I printed the model on Prusa XL, using Prusament Galaxy Black and Lipstick Red. It's printed at 0.15mm layer height without supports and with brim.
Make sure to get the stand from the original model
The author remixed this model.
Added MMU shading.