Psychodelic patterns were not in fashion in 1964. Tom Goodfellow was still head of the company and he wanted to stick with gray/orange, so the modernistic "swoop" was born. New York was absolutely crazy about the opening of the World's Fair at that time. A little history here: Robert Moses wanted the Fair to be his crowning achievement in the east. He was a promoter who was as devious as he was brilliant. So much money was poured into the Fair by exhibitors, it would have been almost impossible to believe it would bust. By 1965, the Fair started to go broke, and all manner of exhibit wore down and were cut, poorly repaired, and starting to close. Still, the railroad had decorated the new cars and locomotives in what remained an eye-catching scheme, supporting the LIRR's colors since 1955. No need to change them, and if the "Unisphere" decals on the MU cars lasted, they were a reminder of history.
Blue and yellow paint didn't hit until 1968, with the arrival of the L2 C-420's. I think those of us who were true LIRR fans were sincerely upset by the migration to blue and yellow. When the FA's came on line, the M1 cars had been introduced, and painting everything to platinum and blue commenced. At least one MP54 got the treatment, too. Platinum/blue was supposed to show the fleet was modernized, which it really wasn't. New services and new methods of employing old equipment were coming online.
Had the ALCO fleet remained in service, the blue and white swoop might not have happened - period. I understand why the company phased out the ALCO's, but I've always felt they could have keep the C-420's rolling with parts still readily available.