Answer: Red.
Explanation: This specific color of M&M's was discontinued for it being linked to cancer in a 1971 Russian study. The synthetic dye used to color the candies was called FD&C Red No.2 or amaranth. Shortly after the FDA issued its ruling, red M&M's disappeared. However, consumers had never faced any risk from red M&M's, which is most likely why it's back today.
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