What you are noticing
A good way to test this is to look at a bright red object, cover one eye, then the other, and compare. If the red looks strong and vivid through one eye but duller, pinker, or greyer through the other, you are describing colour desaturation, and a loss of red in particular is called red desaturation. It is a subtle symptom, but a meaningful one, because it tends to reflect how well the signal is travelling from that eye to the brain rather than anything you can see on the surface of the eye.
Why the optic nerve comes first
The optic nerve is the cable that carries the visual signal from the eye to the brain. When it is not working as well on one side, that eye sends a weaker signal, and the result is a very particular pattern: colours look washed out or less saturated, red especially, brightness fades a little, and contrast falls, even though the eye itself looks completely normal.
The causes to consider are the optic nerve conditions as a group. Optic neuritis, an inflammation of the nerve, tends to affect younger people and can bring discomfort on moving the eye. Glaucoma is common and often affects the two eyes unequally. A reduced blood supply to the nerve tends to affect older people and comes on suddenly. Because these range from the slow and silent to the sudden, washed-out colour in one eye deserves a proper look.
The sign an examiner looks for
Alongside asking you about colour, an examiner will check for a relative afferent pupillary defect, or RAPD. Using a torch moved between the two eyes, they watch the pupils: an eye whose optic nerve is not conducting well reacts less briskly, and its pupil may appear to widen as the light reaches it. It is painless, takes only moments, and is one of the most reliable objective signs that the optic nerve on one side is underperforming.
The main alternative to keep in mind
Not every colour difference is the optic nerve. A cataract clouds and yellows the lens, muting colour in that eye, which is why people often notice colours looking strikingly brighter and bluer after cataract surgery. The distinction is usually in the pattern and pace: a cataract dims the whole image and develops slowly, whereas an optic nerve problem tends to strip colour and contrast in particular, and can come on more quickly. An eye examination separates the two with ease.
Gauging the urgency
A gradual change in colour between the eyes warrants a routine but definite eye examination. A sudden loss of colour in one eye, particularly with discomfort on moving the eye, or in an older person, should be treated as urgent, as it can reflect optic neuritis or a reduced blood supply to the nerve. If in doubt, treat a sudden change as urgent and seek same-day review.
Common questions
What is red desaturation?
It is when a bright red object looks duller, pinker, or greyer through one eye than the other. It often points to a problem with the optic nerve on the affected side.
Does washed-out colour in one eye mean optic neuritis?
It can be one cause, but not the only one. Glaucoma and a reduced blood supply to the nerve can do the same. An examination, including a check for an RAPD, finds the cause.
Could it just be a cataract?
A cataract can mute and yellow colour, but usually dims the vision generally and slowly. A sudden loss of colour, or of red in particular, points more towards the optic nerve.
