After converting from Revit to IFC, model has different colors (or maybe textures), why is it so?
This is likely related to the fact that IFC and RVT files are handled differently in the Forge Model Derivative service. Revit files are processed using Revit, and IFC files are processed using Navisworks. Note however that this behavior will change in the near future: https://forge.autodesk.com/blog/model-derivative-ifc-pipeline-call-change.
