Hi!
You can convert and export the FBX files to GLB using Blender. You can do this quite easily through the Scripting workspace with a small script generated with the help of AI.
Regarding color customization, it uses a mask texture in the shader. Once you take a look at the UVs, textures, and how the material is set up, it should be quite straightforward to recreate the same system in Godot :)