Have you ever received any trauma to the eye or forehead or head? Is there any other symptom in your eyes such as haloes, floaters? If you neither received a trauma nor suffer the symptoms above, then you are likely to develop the symptom that one eye is brighter than the other because of ocular migraines or silent migraines. Ocular migraines manifest no symptom of headache, but they present some eye symptoms, including eye strain, temporary blurry vision, halos in front of eyes and seeing brighter in one eye than the other one. You can take Flunarazine for six months or so to decrease the symptoms of migraine.
It is hard to tell just based on your introduction. Even if seeing a doctor, you still have to provide more information as basis of diagnosis. Normally speaking, the two eyes we have are not completely same with each other. It is very common for two eyes in different conditions. If it is a sign of eye disorder, you might find out more information or symptoms. For example, conjunctivitis might occur to one eye or both. If you got conjunctiva infected in one eye, it is not as bright as the other healthy eye. It also develops redness, itching, teary and grittiness. So you should check carefully if there is any other symptom accompanied. Take an examination of eyes to see if anything wrong.