Depending on the file type there are varying options for overriding materials in the host project. I've included a few examples below...
For Revit links, you cannot override the materials in the host project unless you physically update the linked file material. The linked file material definitions and assignments will determine the material appearance in the host project.
If you need to transfer Revit materials between projects use Manage > Transfer Project Standards > Materials.
CAD Imports \ Links
After linking the CAD file you can override materials under Manage > Objects Styles > Imported Objects.
Look for the name of the CAD file and expand to see individual layers. You can remove layers default material assignment if needed by clicking the "..." button > No Material.
Assign the desired Revit materials to the layers and they should appear in the project views.
For most consistent results when importing SketchUp models directly into the project I would recommend the following workflow:
1. Ensure all SketchUp objects are on specific layers.
2. Explode any groups or components prior to saving a copy of the SketchUp file. This will ensure layers inside the groups \ components are assigned properly.
3. Set the SketchUp model to view Color by layer under the Layers window.
4. Save a copy of the file. Then in Revit, Insert > Import CAD > SketchUp Files.
5. Set Colors > Preserve. Set Layers to Specify...
6. Un-check (Default) and Layer0 [unless you have content on layer0]. This will ensure only the necessary layers are included. Click OK.
7. Navigate to Manage > Objects Styles > Imported Objects. Assign Revit materials to the necessary SketchUp layers.
I've included a short video example below [no sound]:
For additional information regarding SketchUp files see the Revit Help file here. Note you can also import the file into a family for additional customization.