If you would like to create a multi color interior, first you must select your two tone design.
A two tone design is the area you will be applying your secondary color to. It tells us where you want the secondary color to be.
Most patterns will have 5 or 6 two tone designs available (inserts, combo, body, centers, faces, wings).
Make sure to use the design guides in the colors and diagrams tab to view the available two tone design options for the specific pattern you are working with. These are 2D schematics which illustrate where the secondary colors can be applied.