1968 MOPAR paint codes are a single letter.

Nothing was Road Runner specific, Road Runner was simply a Plymouth B-body.
The Plymouth B-body paint choices are in the pic I posted above, the color codes have the red square around them.

STANDARD available B-body paint codes: A, B, E, F, G, J, L, M, P, Q, R, S, T, U, W, X, & Y

Some aftermarket paint charts will show the color codes as: AA1, BB1, EE1, etc. - this is simply showing that the car is painted a single color "roof & body". They also use brand specific names such as "poly" or "irid.", these are paint manufacturer's names for metallic colors & Chrysler Corp. never used that nomenclature.

QQ1:
Q = roof paint "electric blue metallic"
Q = body paint "electric blue metallic"
1 = mono-tone paint style

WM2:
W = roof paint "sable white"
M = body paint "turbine bronze metallic"
2 = two-tone paint style

FWIW - Sherwin-Williams "COLOR MANUALS" show the actual names the manufacturers used.