You have imported a model from your 3d modeling software.  When you use the Material Editor you click on part of the model and Lumion selects all or most of the model.

You find that you can only apply one material to all of that part of the model.  

However you want to select just a single surface or smaller part of the model as you are able to in your 3d modeling software and edit the material.

Here's Why:

To make it much quicker to apply or update materials and to optimize the model for performance in Lumion, all surfaces with the same material name are combined.

In each of the main model import guidelines there is this section:

Why can't you assign a Lumion material to individual surfaces?

• Lumion will always combine surfaces that use the same material in [your 3d modeling software]. If you want to assign a Lumion material to a specific surface, you must assign a unique material to that surface in [your 3d modeling software] before Importing or Re-Importing the model.

Here's an example (using SketchUp):

This model in SketchUp has all surfaces with one material. You can select different surfaces in SketchUp.

(Source model changed to one material for this example.)

But in Lumion when you use the Material Editor you are unable to select the cushion or the base of the chair to change them.

All surfaces wherever you move the mouse to or click have the same 'Wood Veneer 01' name.

In SketchUp you need to select some surfaces and apply a different material to them:

In Lumion, after Re-Importing the model, it now looks like this:

Now you are able to select parts of the chair and apply different materials in Lumion.

When rendered:

[model credit: by John Luttropp, model: Frank Lloyd Wright Usonian Automatic Hassock, Trimble 3DWarehouse] 

See Also:

• Knowledge Base:  Model import guidelines for SketchUp
• Knowledge Base:  Model import guidelines for Revit and Revit LT
• Knowledge Base:  Model import guidelines for ArchiCAD
• Knowledge Base:  Model import guidelines for Autodesk AutoCAD
• Knowledge Base:  Model import guidelines for Rhino
• Knowledge Base:  Model import guidelines for 3ds Max
• Knowledge Base:  Model import guidelines for Vectorworks
• Knowledge Base:  Model import guidelines for Allplan