[Thommo (OP)]

When I walk the dogs in the dark morning, with my head torch I can see their eyes reflect with a bluey white colour, and the pair of of foxes in the church grounds that have a yellow reflection.  There don't seem to be any cats about, but I know from flash photography that people have red reflections.

Why do different animals have different coloured eye reflections?

[RD]

Some animals have a layer at the back of the eye called the "tapetum lucidum" which improves night-vision, that's responsible for the colour of eye-shine other than red.

[Thommo (OP)]

Thanks, but why should it be a different colour for different beasts?

[Colin2B]

The effect appears to be a form of iridescence, in other words caused by the reflected light from the front and rear of the layer interfering with each other. In that case the thickness of the layer would affect the colour.  It should also be affected by incident and viewing angles, but I suspect the curvature of the retina reduces that effect. The iridescence effect can be seen in thin layers of oil on the surface of a puddle.

Edit, have just read that you can get different colours with different viewing angles.

[evan_au]

The red-eye effect in humans comes from the blood supply in the back of the eye.

In the vernacular, it is exacerbated by excessive alcohol and/or insufficient sleep, although this affects a different part of the eye...