From working through the Motor Actions from 1976 the change in paintjob happened between Ruapuna 7 Feb and Pukekohe 7 March. the white front remained until Bay Park at Labour Weekend in October, the loss of Marlboro sponsorship could be accounted for from any of the following,
1; Red had said he wanted one more title tilt, so 76/77 may have been his last season anyway,
2; The talk that the top class would drop from 0-6000 to 5000, or
3; At Ruapuna, there had been issues regarding sponsorship ie; Rothmans attempted to stop Red from racing, so as Peter Stuyvesant were the overall series sponsors for the 76/77 season (Rothmans/BAT) but also Marlboro sponsored the races of the visiting Australians in Oct/Nov 76 and Red and been a vocal opponent, particularly of the legality of the Corvair. So a split by mutual agreement to avoid any animosity.
Now the Marlboro track day was in May of 76 as well.