Food coloring
Green ketchup, pink margarine, blue french fries, now this. I'm not sure I'm ready for purple carrots.

Did you know, by the way, that you can get M&Ms custom-packed in any of 21 different colors? (At $3.99 per 8 oz bag, it's not something you'll want to do often, but it's an intriguing option for a special event.) Sorta takes the edge off the "Vote for a new color", doesn't it?