In Revit you can apply a material to a surface such as paint onto a wall or carpet onto a floor.

Applying Paint

The Paint Tool allows you to apply any material you have loaded into the project so you will firstly want to make sure you have the material loaded into your project

1. Go to the Modify tab in the ribbon bar.
2. Select the Paint button from the Geometry Grouping.
3. After pressing Paint the material browser will appear with a list of all your materials.
4. Select a material and click on the surface, this will apply the material to that surface.

Conclusion
The paint tool is really useful for applying materials to surfaces without the need for editing the structure of walls and floors and having to create a layer.