Only one eye is red in flash photo, the other white-ish. Is that a sign of brain tumor in an adult?

Doubtful. . While loss of the light reflex can rarely indicate there is a mass in your eyeball, brain tumors will typically not cause this. Brain tumors can cause swelling behind the eyes which should not affect how your eyes appear in a photograph. Loss of the light reflex and swelling behind your eyes can both be easily evaluated by visiting your optometrist.