According to "The Pigment Parade" by Lorraine Shelton,  

 The pigment cells are responsible for giving our cats colored eyes as well as colored skin and fur. An eye without these cells is blue. If the pigment cells have marched down from the neural crest far enough to give the eyes color, chances are they have progressed far enough to provide sound hearing as well. But remember how some bicolors are assymetrical? The same thing can happen in whites, causing odd eyed cats or deafness in one ear. I have seen bicolored cats with eyes that are blue on the side towards the middle of the face and copper on the side towards the outside of the face. Now THAT is an odd-eyed cat! As breeding programs consistently produce bicolored and van cats with more and more white on them, blue and odd eyes will increase in frequency in these programs. 



Taking advantage of the polygenes influencing other programs can also benefit someone wishing to increase the incidence of odd eyed whites in their lines. I have heard a theory proposed that adding bicolors to a program will increase the incidence of odd eyes. I think this is possible... if ASSYMETRICAL bicolors are used. Bicoloreds with perfectly even V blazes may even DECREASE the odd of getting odd eyes, in my opinion, because these cats reinforce the trait of symmetry.

PERMISSION WAS OBTAINED FROM LORRAINE SHELTON TO USE THE ABOVE INFORMATION.  BELOW IS A LINK TO HER SITE, "FEATHERLAND CATTERY".

 http://featherland.net/CATS.HTM