Cataracts are formed when the lens inside the eye gradually changes and becomes less transparent (clear), making vision mistier. More information at Cataract page.
Cerebrovascular diseases are conditions caused by problems that affect the blood supply to the brain. One of the most common types is stroke, which can cause loss of vision in one eye or on one side of the field of vision
Effects can vary: there are extremely rare cases where people are unable to see any colour at all, but most can see as clearly as other people but are unable to fully see red, green or blue.