How did Smarties get their slogan, "Do you eat the red ones last?"?

I heard because the red dye they used cause cancer. That can't be right. A company would never draw attention to that. So why is it their slogan?|||Probably, my opinion, because lots of people, like my mom, like to eat them color by color and watch the color change, and red always seems to be the last one. No idea if that's the real reason, just a guess.