How to edit & apply textures from Retera's Model Studio? (Reforged)

Level 1
Mar 24, 2020
Hey all, been having some fun geoset merging with Retera's Model Studio (v0.04.2020.03.03) and I'd like to change the character's hair colour. I understand I need to extract a texture, then edit it in another piece of software, and then re-apply the texture... but it's the 3rd (and possibly 1st) step in that process I'm unsure about. Can anyone help?

So I've cobbled this chap together and I'd like him to keep his hair style but have black hair:


I go Tools -> Edit Texture and I can find his hair texture, so far so good:


I can then export it to something editable like a PNG (is that correct?)

...but then if I edit the texture, and re-import it, or use the "Replace Texture" button I can't figure out how to actually apply the texture to his hair. It just goes matte white/"missing texture". Does anyone know what step I'm missing / is the process actually different to what I'm doing?

Been a long time (15 years?) since I've done any WC3 editing and seems a lot of the old tutorials don't apply to Reforged - so any help most welcome! (And I'm sure some others would love to hear it too).