I tried finding this info and even checked the classic opel registry. It would be nice to have a thread that compiles all the GT color codes, that appear on the firewall plaque. The info should be broken into exterior and interior codes. I've been checking a bunch of info I have, and I've compiled the following lists. If anyone has info that contradicts the following, please share it. This is rather difficult work due to Opel changing codes and different sources giving slightly different info. There are paint codes I have that don't apply to the GT if anyone is wondering. Some paint code directories refer to metallic paint as poly.
Exterior Colors - Factory Code, Name, Plaque Number
1969 Colors
15 - GT Metallic Green / Forest Green - 336
BB - Sand Beige - 448
CC - Sun Yellow - 450
DD - Regal Gold Met. - 447
HH - Strato Blue Met. - 235
JJ - Rallye Red - 520
PP - Spring Green Met. - 338
RR - Brilliant White - 451
UU - Metallic Silver - 135
1970 Colors
16 - GT Chartreuse - 406*
BB - Sand Beige - 448
CC - Chrome Yellow - 446
DD - Aztec Gold Met. - 404
GG - Copper Bronze Metallic - 507
HH - Strato Blue Met. - 235
JJ - Flame Red - 508
PP - Spring Green Met. - 308
RR - Brilliant White - 451
SS - Sunburst Yellow - 407
UU - Metallic Silver - 135
WW - Rallye Orange - 513
ZZ - GT Chartreuse - 406*
* Some sources have this as ZZ, others as 16.
1971 Colors
CC - Alpine White - 416
DD - Strato Blue Met. - 235
GG - Aztec Gold Met. - 404
HH - Jade Green Met. - 304
PP - Astro Silver Met. - 135
QQ - Rallye Gold - 411
RR - Flame Red - 508
TT - Antique Bronze Met. - 412
XX - Fire Glow Orange - 529
YY - Chrome Yellow - 446
1972 Colors
CC or RR - Alpine White - 416
DD or HH - Strato Blue Met. - 235
HH or PP - Jade Mist Met. - 303
QQ or SS - Rallye Gold - 411
RR or JJ - Flame Red - 508
TT or GG - Antique Bronze Met. - 412
XX or WW - Fire Glow Orange - 529
YY or CC - Chrome Yellow - 446
UU - Astro or Grecian Silver Met. - 135
1973 Colors
CC or RR - Alpine White - 416
DD or HH - Strato Blue Met. - 235
HH or PP - Jade Mist Met. - 303
QQ or SS - Rallye Gold - 411
RR or JJ - Flame Red - 508
TT or GG - Antique Bronze Met. - 412
XX or WW - Fire Glow Orange - 529
YY or CC - Chrome Yellow - 446
SS - Grecian Silver Met. - 135
Interior Colors
Black - 751
Buckskin (Off White) - 721
Red - 741
Tan Dark / Brown - 701 (Very Rare)
Tan Light / Beige - 702
Blue - 711 (Extremely rare, photos are the only evidence I have that this exists)
Exterior Colors - Factory Code, Name, Plaque Number
1969 Colors
15 - GT Metallic Green / Forest Green - 336
BB - Sand Beige - 448
CC - Sun Yellow - 450
DD - Regal Gold Met. - 447
HH - Strato Blue Met. - 235
JJ - Rallye Red - 520
PP - Spring Green Met. - 338
RR - Brilliant White - 451
UU - Metallic Silver - 135
1970 Colors
16 - GT Chartreuse - 406*
BB - Sand Beige - 448
CC - Chrome Yellow - 446
DD - Aztec Gold Met. - 404
GG - Copper Bronze Metallic - 507
HH - Strato Blue Met. - 235
JJ - Flame Red - 508
PP - Spring Green Met. - 308
RR - Brilliant White - 451
SS - Sunburst Yellow - 407
UU - Metallic Silver - 135
WW - Rallye Orange - 513
ZZ - GT Chartreuse - 406*
* Some sources have this as ZZ, others as 16.
1971 Colors
CC - Alpine White - 416
DD - Strato Blue Met. - 235
GG - Aztec Gold Met. - 404
HH - Jade Green Met. - 304
PP - Astro Silver Met. - 135
QQ - Rallye Gold - 411
RR - Flame Red - 508
TT - Antique Bronze Met. - 412
XX - Fire Glow Orange - 529
YY - Chrome Yellow - 446
1972 Colors
CC or RR - Alpine White - 416
DD or HH - Strato Blue Met. - 235
HH or PP - Jade Mist Met. - 303
QQ or SS - Rallye Gold - 411
RR or JJ - Flame Red - 508
TT or GG - Antique Bronze Met. - 412
XX or WW - Fire Glow Orange - 529
YY or CC - Chrome Yellow - 446
UU - Astro or Grecian Silver Met. - 135
1973 Colors
CC or RR - Alpine White - 416
DD or HH - Strato Blue Met. - 235
HH or PP - Jade Mist Met. - 303
QQ or SS - Rallye Gold - 411
RR or JJ - Flame Red - 508
TT or GG - Antique Bronze Met. - 412
XX or WW - Fire Glow Orange - 529
YY or CC - Chrome Yellow - 446
SS - Grecian Silver Met. - 135
Interior Colors
Black - 751
Buckskin (Off White) - 721
Red - 741
Tan Dark / Brown - 701 (Very Rare)
Tan Light / Beige - 702
Blue - 711 (Extremely rare, photos are the only evidence I have that this exists)