Revit exports FBX files in a very weird way, the materials seem to be in a non-standard format. Third-party software are not able to read the materials (even unity or unreal engine). Only 3dsmax is able to. It's probably a move from Autodesk to force the use of 3dsmax.
To workaround that limitation, you can use the free Twinmotion FBX exporter: https://www.unrealengine.com/en-US/twinmotion/plugins?lang=en-US
You don't need to use Twinmotion at all. Just install the exporter, and it will give you an FBX that Transmutr can read properly with all materials and textures.