There are 10 two color combinations for land in magic. Black-white, black-blue, black-green, etc. For any one color, 4 of those 10 will contain that color. I.e. four of these two color combinations contain black. That's almost half.
My question, then, is why not run a rainbow (5 color) deck with twenty dual color lands, two from each color permutation? I assume for the sake of simplicity a 20 land 40 non-land split but the principal is the same.
One of the arguments against rainbow decks is that you can easily find yourself in a position where you don't have the land you need. But in this construction, 40 percent of lands will give you any single color, which seems pretty good to me. Am I missing something?