This time, I was wondering if I can use Gryning's beautiful 4t2 conversion of Bedazzled Facepaint from RoM as a witch default.

If so, could I use different colours for good and evil respectively without issue, and how do I replace the files?

Yes, you can - but you need to convert it into a skin overlay first. I suggest you find a default replacement witch overlay for each of the witch overlays and replace the face textures with the ones in the link. You can use different colors for good/evil.

You do need SimPE to extract the textures and to open up and edit the files. You also need the DDS utilities for SimPE for getting the best result.

Tutorial (and DDS instructions)
http://www.modthesims.info/showthread.php?t=194390 (shows how to make default skins, it's old but the method still kinda works, you'll just want the witch skins instead. You can skip step 2, instead extracting the makeup directly with SimPE - open the file, click the first TXTR, an choosing "extract" down in the Plugin View window, and repeat if there are more TXTRs)

Here's a couple good/bad witch overlays you can use as bases for the tutorial (you'll want to replace the body textures with an empty 1024x1024 png if you just want the face markings)
http://ohbehave007.blogspot.com/201...ch-overlay.html
http://ohbehave007.blogspot.com/201...ay-default.html

If anything is unclear, just ask

Thank you so much!! The entire process was so much easier than I ever thought possible.

(Apologies for the horrible screenie; I was in fullscreen and just really excited that it worked out.)

Looks good