The dealer brochure and the shop manual indicate the '66 Riviera was available only with the 425. Reference to a 401 in the '66 Riviera is a goof.
Dealer brochures show the 425 was red in '66 only. Not all 401s were blue-green. Owners may have installed an engine from a different year, resulting in a different color. Too some may have repainted motor if they didn't like the red. Others may not be aware red would be correct.