'68 & '69 white interiors differed, with the '68s not having white door or quarter metal, but the accent color instead. In '69 those areas were changed to white. A '68 headliner would be white, '69 black, with matching moldings at side & back glass. Dash, A-pillar, belts, carpet, kick panels, column, horn pad, package tray & console match the accent color both years. Both my white interior cars had black accent - '68 Charger & '69 Roadrunner.
I don't know of any separate option code for door/quarter color. A '68 GTX white interior/white metal would make sense as an option code, a convertible deviation, or if the RS models were trimmed differently from RM & RP.