Vasoconstrictor. You can shrink conjunctival vessels with visine or equivalent over the counter drop, but should not use regularly due to rebound redness and irritation.
Not too much : (If your eyes are red, then diagnosing and treating the cause of the red eyes is helpful in making the eyes more white. However, if the eyes are yellow (and it's not due to a liver problem, in other words, jaundice) then there's no tried and true method to whiten the eyes. A healthy diet and exercise are your best shot at that.