Why do my eyes turn red?

Increased blood flow. Because the surface of the sclera (the white part of your eye) is thin & translucent it appears red when blood flow is increased. Blood flow increases to the cornea with infection, irritation, or allergy. It also increases with autoimmune diseases affecting the eyes. Persistent redness of the eye, especially in the presence of pain or eye discharge, requires a doctor's prompt evaluation.
Conunctival redness. The sclera is white. It is never an other color (normally). The covering of the sclera is the conjunctiva which has vessels. It is these vessels that become dilated in inflammation or conjunctivitis. The cornea is avascular and gets oxygen from the air, tears, etc. If something is irritating the eye, the conjunctival vessels become dilated, and the eye will appear red. Depends on the cause!
Many possible causes. There are many causes of redness of the eyes. Several types of inflammatory conditions, allergic reactions, detergents, chemicals, and sometimes bacterial, viral or fungal infections can cause redness of the eyes. Have it evaluated by a professional and get the appropriate treatment.

Related Questions

Why do my eyes turn red after I take my sunglasses off?

Depends. It really depends on how long it takes to become red and where you are when you take the sunglasses off. For example, if you are outdoors, and it happens rather fast, then your eyes may be dry, and the sunglasses were protecting the eyes from drying up. Regardless, something is going on with your eyes, and you should see your eye md for evaluation.

Why do my eyes turn red after a work-out? White area of the eye turns red.

Sweat. You are probably having sweat, which is much saltier than normal tears drip into the eye. This hypertonic (salty) solution can inflame the conjunctiva. Try exercising with a towel, wrist or headband to keep the sweat out of th eyes. If they still look irritated after a workout, put an artificial tear drop in the eye when you are finished.

Why do my eyes turn red and watery when I use makeup or eye shadow?

Allergy or Irritant? This could represent a contact allergic reaction due to preservative, and fragrances in the cosmetics. Even "hypoallergenic" products can cause a reaction, especially if your tissues are already irritated. It could also just be an irritant reaction due to a foreign product near the eyes. Either way I would discontinue the products and let the area return to normal.
Possibilities. You might be having an irritant reaction or you could be having an allergic reaction. Visit an allergist in your area to discuss this issue. They might recommend patch testing to identify if you have a sensitivity to an ingredient in the makeup.

Why do my eyes burn and turn red in certain environments?

Irritation. The environment is filled with things that can irritate your eyes. Dry eyes (insufficient tears) can make these things cause more irritation as they are not rinsed from the surface of the eyes. Some specific things bother some people and not other people. Allergies are an example of these. The hallmark of allergy is eye itch. Avoid your irritants - see your doctor if this is not enough.