Revit needs to generate .dwg drawings according to a CAD standard, which requires placing Walls on different layers and assigning different colours to them according to specific grouping criteria, e.g:
Groups of Walls and AutoCAD layers they should be placed at required by the CAD standard
Layer Modifiers in DWG/DXF Export Setup table allow placing objects of the same Category (Walls in this case) on different AutoCAD layers based on values of some of their properties, but the set of those properties is very limited and their values are usually hardcoded in Revit. The only Wall Type property, which can have custom defined values and can be used as Modifier is Fire Rating. Therefore the Wall Types should use the Wall grouping criteria from the table above as values of their Fire Rating property. With those values defined and the configuration of the DWG/DXF Export Setup as shown below any number of our Wall groups can be exported to their respective AutoCAD layers.
DXF/DWG export setups for Walls with Fire Rating as Layer Modifier