I don't know if it would relate....but in humans there's supposedly only two colors, brown and blue. It was thought there was green too, but it's been proven that they are actually blue eyes. And reading the eyes to determine the health of an individual is called iridology. If a blue eye on a person were to go green it would indicate that the person is consuming something that the body is unable to process and it gets stuck in the body for a little while till it can find it's exit.
Maybe horses experience the same thing? I really don't know. But if so then perhaps there's something that he's eating that is slightly toxic to him? I don't know, but it's something to at least consider and possibly rule out. Perhaps some semi-toxic plants in the pasture just popped up, or maybe something is being absorbed through the skin. Just some ideas to ponder for ya.