My 64 was Diplomat Blue with a white custom interior. Chevy called theirs Daytona Blue. Pontiac and Olds also had the same color with different names. Not sure about Cadillac though. Chevy eventually changed the name to Deepwater Blue but it was the same color.
If you put a 1964 Diplomat Blue Riviera along side a 1965 Midnight Blue Riviera, you would be hard pressed to tell the difference.