Re: Why are certain animal's nicitating membranes colored?

Hi – I think there is confusion in your question between nictitating 
membranes and colour vision. The nictitating membrane is common in cats 
and dogs, and is commonly called the third eyelid. In dogs, it sweeps from 
the inside corner of the eye up and across the front surface as an added 
form of protection. The small red triangle in human eyes is our atrophied 
version.
Other than to protect the surface of the eye, this membrane has no role in 
vision itself, and is not responsible for the yellowish reflection you get 
from dog (and cat) eyes at night. I’ll say more of that in a moment.
Dogs are not colour-blind, but their eyes and their colour vision differ 
significantly from ours. Firstly while we have three-colour vision (red, 
green and blue) due to our cones being specifically sensitive to the red, 
green and blue wavelengths of the spectrum, dogs have only two and tend to 
see best in the yellow to blue parts of the spectrum.
In our eyes the cones are centrally located and tightly packed in the 
fovea, with the rods (not specifically sensitive to colours) spread more 
evenly across the retina. Dogs have their two cones spread more evenly and 
are much more sensitive to light and dark differences – they are more “rod 
dependent”. A small percentage of human males have a colour “blindness” 
due to having only two significant cone sensitivities and it leads to red-
green blindness. It seems dogs are much the same and cannot easily see red 
against a green background. They make up for it by very high sensitivity 
to movement (rods).
As for the colour reflected back from a dog’s eye – as you rightly point 
out this is from the back of the eye – the retina (not the front, where 
lies the nictitating membrane). In humans we see red eyes in flash 
photographs when the flash is too near the axis of the exposing lens. This 
is a reflection of the redness of our retina, which has lots of blood 
vessels close to the surface. Dogs, as well as an extra eyelid, have a 
highly reflective layer of cells behind the top half of the retina that 
reflects light back to the rods and cones, increasing detection limits and 
thus enhancing night vision.. Yes, dogs and cats see better than we do in 
the dark. This layer is responsible for the shine of the dog's eye in the 
dark in response to light, and is thought to shift the colour of the light 
to be more aptly noticed by the rods, enhancing contrast – but giving no 
colour information. This layer is called the tapetum lucidum and is common 
to a lot of mammals, whose eyes shine yellow or green at night.
So, having said all of that – I’m not at all sure that the nictitating 
membranes are coloured – if they are, it isn’t significant to the effect 
your question goes on to discuss.
Thanks for the enquiry.