Select a layer and click the Material ID icon at the bottom of the layer stack to add a Material ID Mask to the selected layer.
Adding a Material ID Mask to a layer allows you to apply a mask only to a selected part of the mesh and not anywhere else.
Apply Material ID Mask from the Color List
You can select any color from the list where you want to add a specific texture on that particular part of the mesh.
Apply Material ID Mask through the Color Picker
While the Material ID Mask is selected, by holding down “Q” you can switch from your current render mode to the Material ID render mode, where you can easily toggle color IDs on and off with the color picker. Releasing “Q” takes you back to the previously selected render mode.