As far as I know, any information for first gens that purport to state 1 of xxx factoring in color, style and engine are using interpolation.

For instance, 3 out of 10 1968 Firebirds were Verdoro Green. 2 out of 10 were convertibles. 2 out of 10 were 400s, 4 out of 10 400s had 4 speed (all numbers are hypothetical). If you have a 1968 Verdoro Green Firebird 400 convertible with a 4 speed manual transmission and there were 80,000 built, you could hypothetically say your car is one of 4800 Verdoro Green convertibles, and one of 6400 400 4 speed Firebirds (coupe and convertible). By taking 6% (color/body) of the 6400 400 4 speed, or 8% (drivetrain) of the 4800 green converts, you could say your car is 1 of 384. Is it? Who knows. Does that make it more rare than an original '69 Trans Am? By the numbers, possibly yes. But that does not increase its value.

Once computerized records were retained in database format, it became simple to query the database to determine how many of this and how many of that were combined.

I do not have access to sufficient factory data to even begin to sample color ratios. I hope PHS is datalogging as they process PHS reports, including cars not requested.