Subconjunctival Hemorrhage: This condition is characterized by a sudden appearance of blood under the conjunctiva, which can cause eye redness without itching, pain, or swelling. It often results from a burst blood vessel and can be triggered by coughing, sneezing, or straining.

Dry Eye Syndrome: Although dry eye can sometimes cause discomfort, in its mild forms, it might only present with redness without significant pain or itching. It's a common condition, especially in environments with low humidity or in individuals who spend a lot of time looking at screens.
Conjunctivitis (Viral or Bacterial): While conjunctivitis often presents with discharge and sometimes itching, mild cases might only show redness. The absence of itching and pain doesn't rule out conjunctivitis entirely, especially if the condition is in its early stages.
Allergies (Mild): Even though allergies typically cause itching, a mild allergic reaction might only result in redness without the other symptoms.

Acute Angle-Closure Glaucoma: Although this condition typically presents with severe eye pain, blurred vision, and sometimes nausea and vomiting, in some cases, the pain might be mild or absent, especially in the early stages. It's crucial to consider this diagnosis because it's an emergency that requires immediate treatment to prevent vision loss.
Endophthalmitis: This is a severe inflammation of the interior of the eye, usually caused by infection. While it often presents with pain and vision loss, early stages might only show redness. Given its potential for severe vision loss, it's a diagnosis that must not be missed.

Scleritis: This is an inflammation of the sclera (the white part of the eye). It can cause redness and might be painless in some cases, although pain is a common symptom. Scleritis is less common than other conditions like conjunctivitis but should be considered in the differential diagnosis due to its potential for serious complications.
Keratoconus: This condition involves a thinning of the cornea, which can lead to redness among other symptoms like blurred vision. It's more of a structural issue rather than an inflammatory one and is less likely to cause isolated redness without other symptoms.