From the FAQ on m-ms.com:

Question What is the percent of each colour in M&M's® packs? Answer Hello, M&M's® Milk Chocolates are made up as follows: 31% Brown, 19% Red and 12.5% each of yellow, orange, green and blue. M&M's® Peanut and Crispy are made up as follows: 16.6% each of brown, yellow, red, blue, green or orange. Because we make so many at any given time, it is not likely that you will open a pack and get this exact percentage split of colours. To test if this split actually works you'd need about 5,000 M&M's® and that's a lot of M&M's® to eat ! Regards M&M's® -- Mike Stay staym@clear.net.nz http://www.xaim.com/staym