Whats the differance between R4 red and R6 red colors

M=medium price class (Swinger)
E55= 340
R4 R4= bottom and top color, bright red (Dodge), both top & bottom are the same color. Ply R4 is called performance red. Same formula but Dodge calls it one thing and Ply calls it another. Like FM3, panther pink (Dodge), moulon rouge (Ply).
L=low trim grade, 2=vinyl bench seat, X=black.
Dart Swinger, 340, 4spd, black bench seat
Owned the car for 19 or 20 yrs and only have a couple of pics of it. see if we can find one. Nice color, I liked it. Ray