Inch for inch, it is likely that when a male and female walleye are the same length, the male would be the older of the two,  thus more mature and heavier girthed-until later in the winter/spring when the female's eggs get more developed and cause her to outweigh him for a few months.

As far as the color, in addition to variations within a population, it can often also be related to stress-maybe he was hooked deeper or otherwise injured more than she was during the fight? But habitat does influence their color too, so you may be on the right track about them matching their surroundings. (Yellow perch caught in weedy area are much bolder colored than those caught in mud bottoms).