Found the reference, an old booklet "68 Ford Mustang Facts" compiled by the Editors of Mustang Monthly Magazine, no publishing or copyright date.
It notes the following "recommended" accent colors, which may align to standard GT/CS stripes
Black stripe with colors = I, M, N, O, T (notes for M, Wim White, a Gold stripe was used with saddle and nugget gold interiors and only with black interiors on Candy Red cars)
White stripe with colors = A, B, D, F, Q, R, T, U, X (notes Gold stripe used with Raven Black and Highland Green with black, saddle, parchment and nugget gold interiors, gold strip used with Royal Maroon with black or parchment interiors)
Red stripe with colors = M and N (with red interior)
Blue stripe with colors = M and N (with blue interior)
Not sure if its accurate, from a Ford doc or compiled from elsewhere, no mention of a reference.
HTH