Eye color change question... dark to light

I've looked in the memories and there are some tutorials about enhancing the color of someone's eyes or changing a pale eye color (i.e. blue to green or purple), but I was wondering if there was anything out there about realistically changing someone's eyes from a dark color to a light color, like brown or black to blue? By "realistically" I mean making it look natural, not like I obviously airbrushed it myself.

I use Photoshop.