White on white describes the interior as well as the exterior of a car, an airpalne or any other vehichle for that matter. The paint job being white (usually candy paint for a car), and the interior being white (which is almost always leather on any vehichle). In a plane, eveything else is also leather, not just the seats, but also the armrests, the windows, the walls etc. The owner of the vehichle usually tends to design everything white on the white on white vehichle to match the interior and the exterior, like the rims, the wheel caps, and sometimes the grill.
Jay-Z's verse in Hustlin' remix:
Who you haters think you talkin to, I'm the fuckin boss.
White On White G4, hater get lost.