The '69 car did look a lot like FM3.

They also did 'test spray out' colors for the next years models to evaluate them in person. (Like the A15 mentioned above)

The earliest known 1970 car in what appears to be FM3 is the auto show 1970 Challenger R/T convertible & the paint code on it is 999. Did it stir up enough interest to add the color to the lineup? Could be, but we may never know for sure.