Eyeshine

Tails shouldn't be interrupted when he's working late night.

Tapetum lucidum is a retroreflective layer of tissue behind the retina in the eye. Light enters the eye, is detected by photoreceptors in the retina and is then reflected through the retina again to allow more light to be detected. This adaptation aids the night-vision of many nocturnal predators.

And if you shine a light at their eyes, a lot of it reflects back, coloured by whatever reflective substance that particular tapetum incorporates. Foxes have been known to have 'eyeshine' of colours blue, green, yellow, orange, white and red.