I can't help you directly, but I did the paint breakdowns for Camaros. Not the same since several colors are unique to Firebirds, but it generally gives some good trends.
http://www.camaros.org/exterior.shtml#ColorPercentages

I have some data for F-birds but it's small. Nothing to hang your hat on, but it shows that 72 and 73 were about the same (~11%), which is really close to the numbers for 57 and 72 for Camaros.
76 is running about 5%.

Doing a database is a lot of work. I manage the one for CRG and it's at 15,000 cars. The logistics for entering that much data is mind boggling. And it's a 10MB Excel file...

Champagne is a rare color. I've only actually seen it on a full-size car in a junkyard!