Tienes varias tecnicas:
http://www.cofregrafico.com/como-cambiar-color-de-ojos-con-photoshop/#more-113
http://www.baluart.net/articulo/cambiar-color-de-ojos-con-photoshop
http://www.solophotoshop.com/Tutorial-Cambio-de-Color-en-Ojos-Oscuros-c-470.html
Saludos