If the seats are originals, age will be a big factor in getting a good match, if they're new Legendary Auto covers? you stand a better chance matching the color, 70 Charger interior uposltery appears white on it's own, but is actually somewhat "coffee" in color compared to their 68-9 siblings .... for an SEM match use SEM's Super White, I'd recommend a clear coat of SEM's Low Lustre Clear after spraying the white (or any SEM color for that matter, the clear will not alter the color/finish you applied) it helps retain the color, and protects it from dirt, stains, mars, cleaning agents, etc, etc

You can either use SEM Super White in spray can form or have it mixed in pints/qt/gal by any SEM/ automotive paint supplier (preferred) here"s the gram mix formula for your mixer if needed? and a few pics of my 70 Charger interior, door panels/seat covers are Legendary, the upper pads where black dyed with SEM Super White/Low Luster Clear, the arm rest were white NOS, all "hard" seat components and assorted interior/window trim was black originally... the SEM dyed interior pcs still look "fresh" 15 yrs later....

Mike