When you feel it, it may not be normal. You'd better observe your eyes recently. You could use the eye drops with anti-inflammation to make the eyes feel bright. In my own opinion, I suggest you to go to see the doctor and have a full check on the eyes which are very important for the whole body.
No. People's eyes are totally the same, they are different some how in some degree. The difference of color, size and shape of eyes is common, so if one of your eyes is born lighter than the other, don't worry about it. Some people have different color of each eye. What's more, eye condition are changing all the time, many factors can affect your eyes' development. Generally, the differences are harmless, but some times, one eye's change may indicate the illness of that eye and it often comes with other symptom. If you want to make sure your eyes are fine, go see a doctor.
Ok, it seems that you are concerned about this matter. But as far as I know, we simply don't have that problem, and the color of our eyes is all the same. Since the color would not change a lot during our lifetime, let alone becoming different. So you should try to make good use of your eyes and be careful with them. Good vitamin intake and a healthy diet would do you good.!
It depends on how serious is the situation. Is the difference very noticeable? If not, it is normal as no one has perfect symmetry. However if the difference is obvious, there must be something wrong with that darker eye. The possible causes include: ocular migraine, silent migraine, amblyopia, cataract, or some other ocular diseases or nerve diseases. You need to check with your doctor as soon as possible.