This is what I found:I was able to discuss the stripe addition to the new heritage with a person of great authority on the subject. The color is black not dark blue contrary to popular believe. Those is know stated the original gulf color GTs were all painted with black stripes. During design of the gulf color scheme, dark blue was discussed but black was used.
"an original Production Car Record Sheet shows the Light Blue code to be (P030-8013) and the Marigold (P030-3393) both being British Leyland (Triumph) colours"
The gulf colors (the originals):
Berger Ref 1125 Standard Triumph Powder blue
Berger Ref 1465 Triumph Marigold
Powder blue (PPG 12163)
Topaz (orange) (PPG 60812)
Royal Blue for thin stripe between orange stripe and powder blue (PPG 13126)
Also a few paragraphs down in this link talks about the final stripe color being a dark blue. I guess maybe they didn’t know the final color was changed to black.
From the story on the link above:
Gulf had earlier acquired the Wilshire Oil Company of California, whose corporate colors were powder blue and orange and Davis wanted to use those colors. He may have been on to something. The lighter blue and that shade of orange are considered “equiluminant” colors. The human eye has a hard time perceiving the edges of objects when the objects and their background colors have similar luminance. That makes the edges seem to vibrate which give this particular color combination a lot of visual pop. The final livery actually includes a dark blue hairline border around the orange, which reduces the optical illusion and any visual discomfort while maintaining most of the visual impact.