I guess Corporate Blue is darker than Petty Blue, you can see the difference between the Superbird and the dealer service sign and this Petty race car in the ad...
and here is the supposed real story on Petty Blue from Allpar (coincidentally when they went to paint the '59 Plymouth Fury:
Lee Petty wins the first Daytona high-banked oval race and rejoins Chrysler
Lee Petty won the first race ever held at the Daytona Beach high banked oval. He was locked in a three way dual on the final lap. An unusual angle for the finish line camera seemed to show that John Beauchamp had won the race. However, other pictorial evidence indicated that Lee Petty had won by a width of the Oldsmobile's bumper. Finally, after three days, a lap recount was made, and it turned out that not only was it a length of a bumper, it included one full lap. Beauchamp had only turned 199 laps to Lee's 200. Lee Petty was declared the outright winner. Lee reverted back to the trusty 1957 Oldsmobile for short tracks. The 1959 model was to big.
Lee knew the 1957s were too old. He had his eye on a certain make of car that would lead him back to his earlier philosophy that lighter is better. He made a few inquiries and was rewarded with a phone call from Highland Park, Michigan. Within a few hours, 4 "in white" 1959 Plymouth Fury cars were on their way to Level Cross, North Carolina. Plymouth was rather eager to help. They had not recorded a single win in two NASCAR seasons when Lee Petty had left them for the Oldsmobile. Dodge went one full season without any wins in 1958. 2 of the cars were convertibles for Richard, and two of the cars were hardtops ostensibly for Lee. Just about the time that the deal was struck in the spring of 1959, NASCAR announced that 1959 would be the last season for convertible racing. Too dangerous for the driver was their conclusion. Top down racing provided no protection in a roll over. And there were plenty of them on short dirt tracks. With no thought to that, Richard competed heavily in his new Plymouth convertible. Lee responded to the help given him by Chrysler Corporation by winning the very first event that he ran his new Fury in June 1959. After using the Oldsmobile cars for 20 races, the Plymouth's took over, and the Petty organization never looked back.
Petty Blue
Richard's Fury wasn't ready until just before the July 4 Firecracker 250 at Daytona. Both Lee and Richard competed in the hardtop Fury's in that race. Lee lasted 77 laps, only to watch his son make one more circuit and retire on lap 78. However, things like that were not the norm for the Pettys. They prided themselves on being prepared before getting to a race track and it showed. Richard competed in 21 races in 1959. He came in 4th in the convertible division and 15th overall in the Grand National points chase. Doing Plymouth and Petty Enterprises proud, Lee Petty won his third Grand National Championship. The cement that made the organization had Plymouth written all over it. 1959 also marked the first use of "Petty Blue" for the paint on the Petty built cars.
When asked directly about the "Petty Blue" color, Richard related, years later, that indeed, it had been a fluke. Late in the 1959 season, after having to rebuild one of the '59 Plymouth Fury's, Maurice told Richard that they didn't have enough white paint for the painting of the entire car. Being brought up to waste nothing, Richard found some dark blue paint, but not in a sufficient amount to paint the whole car. Richard said they he looked at Maurice, and Maurice looked at him, and they just dumped the two paints into one tub, then proceeded to paint the Plymouth in the color that came out. When they got done, they were impressed themselves with the hue. Lee came back from an out of town race, and marveled at how that color set the car off. Almost electric is what he said. Of course, Richard and Maurice had to scramble to remember the right mix. Once they did, the formula became their secret, even to the point of being patented by a paint company as Petty Blue.
When the 1960 season dawned, their was only one color on the Petty cars. The "electric" Petty Blue. It became their trademark.