Scientists have found that some colour blind people are missing as many as one third of the normal number of specialised light-detecting cells.
Loss of cones in the retina may cause some types of colour blindness
However, apart from colour blindness, the general quality of their sight appears unaffected.
The researchers hope their work will enable earlier detection of eyesight disorders.
The study, by the University of Rochester, is published in Proceedings of the National Academy of Sciences.
The Rochester team used a technique called adaptive optics to study the retina of the eye in much closer detail than has previously been possible.
It was originally developed to help astronomers see more clearly through the Earth's atmosphere.
Lead researcher Dr Joseph Carroll said: "Not only are we excited to show how this method can reveal us living cells in a way never before possible, but it's revealed a mystery with profound implications.
"If a third of the light-receiving cells in your eye are absent and you don't even notice it, it means that when a patient complains to a doctor about waning light sensitivity, then the damage must already be very serious."
People with normal colour vision interpret three bands of light - red, green and blue.
Cells in the retina of the eye called cones contain photopigments that absorb these light bands and send messages to the brain.
People with colour blindness for one reason or another are unable to process these light signals in the usual way because their cones are unable to distinguish properly between colours, such as red and green.
The most common form of colour blindness is trichromatic - where all three cone types are present but one of the cone types does not respond in the normal way.
Dichromatic colour blindness is less common. In this form of colour blindness the person is missing one of the three pigments needed to distinguish between colours.
Until now, it was unclear whether individuals with dichromatic colour blindness had the normal number of cones, but just two types instead of three, or simply lacked a particular type of cone, and consequently had fewer cells to make sense of light signals.
Dr Carroll and his team used a special ophthalmoscope to take detailed pictures of the cones within the eyes of people with dichromatic colour blindness.
In one person the red cones were replaced with green cones, but they still had the normal number of cone cells overall.
In another person all green cones were completely missing.
Surprisingly, this loss of one-third of the retinal cones did not impair any other aspect of vision other than colour.
Ms Catharine Chisholm, an optometrist at City University in London, said while the findings were interesting, the form of colour blindness studied was relatively rare.
However, she thought the retinal imaging technology used was very exciting.
She said: "It's the first time we have been able to see the retina in that kind of detail.
"It does have all sorts of implications for picking up diseases early."