Color change is brought up in this video around the 8:00 mark. The “Boyette” JL8 car was nicely restored but was finished in Rally Green/Black instead of its original Fathom Green/Med Green and was not selling at auction when it was offered in 2013, even though it was matching #’s. https://www.youtube.com/watch?v=yTnV-okOpPE

Black interior

A "collectable" car, like the JL8, is really, a whole different ball game than a rather standard production L35 SS car. The JL8 is going to attract the high end, "investment" group, where a standard "order me an SS" Chevelle, even as a convertible may well not.

With black interior as ordered, I wouldn't have any problem painting the car whatever color I like and not be concerned about resale value. The resale value of convertibles always seems to hold higher than a hardtop anyway and if it's your car, do as you please.

There is absolutely no Chevrolet or Fisher Body records that show how many total 1970 SS Chevelles were in these painted color completions,none.
"Cortez Silver (12,959 built) Note: Silver was a much rarer color according to production numbers noted above."
"Fathom Blue (22,388 built)"
These are guesstimates based on extrapolation,as they didn't keep track of how many colors were assembled back then.

The number by color are true in aggregate for 1970 Chevelles through March 1970. They are not extrapolation but actual numbers that don't reflect total production. The only handy part about them is seeing the hierarchy of colors, as I'd bet they'd be similar or exact by the time production ended in July.

How about if the car was a special paint (- -) car? Would a color change affect the value since there no proof of the original color on the tag?

Some guys see “- -“ as carte blanche for era correct color choice. I personally look at it as an open issue of sorts if there isn’t some forensics (pictures of old paint, paint code formula written on build sheet, etc) backing up the color on the car.

Painting a car to remove ALL evidence of the original color would require a complete disassembly and every bit of original color removed or covered. Without that, there are places the original color can be found and determined from.
Even with a nut and bolt, complete, to bare metal, restoration in a color of the owner choice, as Tom said, without original documentation or historical records, the color could always be in question as original.

My Monaco orange ls6 has nothing on the trim tag thankfully the buildsheet is present a long with speaking with the original owner. After watching that boyette camaro video I wish I'd had him sign something proving it's the real deal

Color change is a touchy subject, but I'll offer my $.02. If it's a high dollar rare musclecar, stick with the original color combo unless 1. It's horrible AND 2. You're worried about resale value.

I had a 70 Ram Air IV GTO, M21, 4.33, manual drums and manual steering, poverty caps. It was Pepper Green/Black from the factory and when I had the restoration done, I went with the factory color. It looked phenomenal. Car won Best Concours Restored at 2013 GTO Association of America Nationals. Later went to Mecum Indy and didn't sell with an $80k reserve. I wonder what would have happened if it was black; I think it would have sold.