According to the M&Ms’ Web site, red M&M’s were pulled in 1976 due to the controversy about Red Dye #2, even though that particular dye wasn’t used in M&M’s. Red M&M’s were brought back in 1987.
The red dye number 2 scare I don’t believe was debunked. I think you are still not allowed to use red dye #2 in the US and probably most other countries with FDA like regulatory bodies. Other dyes were found and are used.