There are a few ways but
Texhammer is the new dat / texture tool
https://korithsmods.blogspot.com/p/tex-hammer.html
Find the dat files via altanaviewer
save the texture out
Open them in gimp or photoshop
recolour
save out as DDS-dxt3 (requires a plugin for gimp or photoshop) or png (pallette) depending on the what file type texhammer tells you the image is. Drop it back into the texhammer in the correct area and save. You have to take care to not ruin the alpha texture (The clear areas of the image for hair outlines etc).
Can also use changetex to save out the dxt3 files then use graphics convertor to split the alpha / main textures and save the 2 out, then edit the main texture and put the main/alpha back together in photoshop / gimp and save it out
If you're careful you can just edit the image and not mess with the alpha at all and save it out as a dxt3 without messing with alpha layers and it will be fine. The proper way is to split the alpha and main image into layers so you can put the alpha back after you're done but that's more involved.
there is a discord channel too i think
https://www.reddit.com/r/ffximodding/comments/9ndxhs/resources/