Like
@69CoronetRT explained, the first letter of the color code on fender tags refers to its model year of introduction. The list is
E 1969
F 1970
G 1971
H 1972
J 1973
K 1974
L 1975
M 1976
P 1977
R 1978
You can see this nicely play out with the Sunfire Metallic colors, that mostly appeared at the start of the 1977 model year:
PB9 Starlight Blue Sunfire Metallic
PF7 Forest Green Sunfire Metallic
PR8 Russet Sunfire Metallic
PT7 Coffee Sunfire Metallic
PX8 Formal Black Sunfire Metallic
Two Sunfire colors were introduced the year before, so they get an M:
ME8 Vintage Red Sunfire Metallic
MG9 Deep Sherwood Sunfire Metallic
The
1973 Fury Special (SPD 531) runs counter to this pattern. It already makes use of KT9 Dark Chestnut Metallic before the start of the 1974 model year, but yet its color code starts with the K for 1974:
So maybe the choice of the first letter depends on how close to the start of a model year a new color is introduced? Also note the late 1977 model year LeBaron, with all its color codes starting with the R for 1978:
PS.
The LeBaron and Diplomat were introduced by late April or May 1977, which puts them very close to the start of the 1978 model year.