My guess on the weight is that they are looking for a certain volume, which happens to come to that particular weight. The colors are not dispensed individually into the bags, the candies come down a conveyor mixed. I don't believe that they produce the colors in even numbers either.
It occurs to me that since I have answered you, I have entirely too much time on my hands today. I'm going outside and getting busy immediately.
M & M’s are available for purchase at craft stores, in large quantities, of individual colors, in individual bags. Don’t know about the weight. You can get all the blues your heart desires!