That I can answer. All color M&M's are dumped into a hopper where a certain number are then dropped into the little bag. To have the colors equal they would have to have six hoppers and the bag would have to stop six times. This would be a waste of space, time and money.
So, if this is the case we can assume that one in roughly every 28,430,288,029,929,700,000 bags of peanut butter M&M's will have all the same color. (26 in my bag so the odds would be 1 in 6^25.)