Consumers can vote online at mms.com, by post or through SMS by texting their name, gender, postcode, age and colour choice. Voters can enter a competition to win a trip to New York to see the unveiling of the winning colour.
Banners and pop-ups will run across mykindaplace.com, and competitions including the New York trip prize draw will feature in the site's Win Bin.
The three-month deal will include pre- and post-campaign market research, and M&Ms digital agency, Grey Interactive, is also creating a dhtml campaign.
M&M's media agency, PHD, is handling planning and buying. Activity will also run on National Magazine Company's Cosmogirl! site.
"To work with such a major brand as Mars, which has real relevance to our audience lets us create effective and fun advertising,
said Charlie Redmayne, marketing director at mykindaplace.com.