I believe it is called 'inverse' but reversal is perfectly understandable and it is caused by the way the bean seedcoat is formed, which is in layers. A seedsaving friend asked OSU years ago on my behalf, when I first discovered this with a cranberry type of bean. If the seedcoat starts to develop with the 'wrong' colouring, then there are then only one (or two) other colours available to form the rest of the seed coat. It develops in layers apparently. When conditions are not favourable, there is a slight increase in this happening. And of course inversed beans will grow into normally coloured beans again (with only a small percentage inversed). Pods with both types of seeds are rarer, but do happen, just like yours.
A blurry photo (sorry) of Gramma Walters is attached, which shows one normal seed (bottom right), 4 fully inverted, one half inverted and one partially inverted seed, ie where only a part of the seedcoat is inverted. Full inversion is the most common form, but with some beans partial inversions are possible.
