Rods do not react to different colors. They see only different shades of grey. That is why we do not see colors when the dusk becomes sufficiently dark. Retinal blood vessels are very thin vessels. They can be affected by many diseases.
Physicians look into the eye to see the retina and the retinal vessels.
The small lamp that the physicians use to see the eyeground is called ophthalmoscope . Choroid Choroid is a thin layer of vessels behind the retina. The outer layers of the retina get their nutrition from the choroidal circulation. The very central part of the retina, the macula also gets its nutrition from the choroidal circulation.