Huh? I didn't know that. I assume you would have to paint the car in one of the original colors, though, to have the interior color change. For example, if the car came with a red interior when the car was painted in <insert specific red paint> you can't expect to get the red interior unless you paint the car in that specific red paint color. Any other manufacturer's red paint will not trigger the interior color change. Painting it black won't trigger a red color interior change, etc.
I guess I've never tried doing this, but how is one to know what the specific paint color is that triggers the interior change? I know about the trick to see all the different paint colors in the showcase section, but they don't tell you the actual name of the paint colors. They just show you the different colors. Are you supposed to just hunt around in the livery editor under the specific manufacturer's paint colors and find the closest match? Some manufacturers have 10 different blues available. Does the livery editor change the interior color of the car as you change through the different exterior colors?