In the 3D view, click on the face that is to contain the attribute with your right mouse button and then on the command Face attributes....

-> The Face attributes window opens (at first empty).

Click on Add....

-> The Face attributes window opens.

Click on the desired attribute class. In this example: SYS_FaceAttributeColor (Class for colored faces)

-> The respective dialog is opened.

Click on Color of face and select the desired color. Color values are automatically calculated and entered. Confirm with OK.

Selection of color

-> The selected face is now colored.

The coloring of the face is yellow according to the respective setting.