The models have baked in materials, so there is no seperate texture file.
If you want to see an example of it, you can download this free model to test it out. Everything in this pack is the same way as this one:
https://reactorcore.itch.io/blaster-gbe-mk1
Technically changing colors is possible within Unity by opening a model to see its materials and changing each color. This will cause all "voxels" of the same color to become that new color. This can be done inside of Unity.