Quote:
The iris is an annulus (or flattened ring) consisting of pigmented fibrovascular tissue known as a stroma. The stroma connects a sphincter muscle, which contracts the pupil, and a set of dialator muscles which open it. The back surface is covered by a two-cell thick epithelial layer, the iris pigment epithelium, but the front surface has no epithelium. The outer edge of the iris, known as the root, is attached to the sclera and the anterior ciliary body. The iris and ciliary body together are known as the anterior uvea. Just in front of the root of the iris is the region through which the aqueous humour constantly drains out of the eye
So, he says making quite a jump in reasoning, external air pressure (think barometers which do exactly the same 'predictive' stuff) can have an effect on eye colour, as when external pressure changes your eyes should adjust their internal pressure, thus making the iris more or less contracted, thus changing the apparent colour. ..