Walleye like many fish species have a large variation in color, both in actual color and hue. There are several reasons for this. Finding a fish that exhibits a color variation noticeably different from other walleye really isn't that rare as far as the species goes. But seeing one like this is a bit unique, such a striking difference according to your description. Occasionally one will be caught with a similar striking difference, but the color will be a dark blue and silver. There may be something else going on with this fish, but most likely it's just a more extreme color variation. Great you got the chance to see it in person.