There is a difference between "whitening" your tooth and "brightening" them. To "brighten" your teeth, you have to focus on the enamel, since dentin is usually covered by enamel and you don't have access to it. To "whiten" your teeth, you must focus on the dentin. There is very little you can do to "whiten" dentin, except to use some kind of oxidizer of pigments. The best is oxygen, in the form of something like hydrogen peroxide or carbide peroxide. There are other "oxidizers", but these are the most common. You cannot "brighten" dentin, since it is usually covered with enamel.

"Brightening" enamel can be done by either removing stains or also to oxidize them. So, most of the "home made" ways either are quite abrasive or demineralize the enamel since the pH is very low (lower than 5.5 for enamel). An example of what NOT to use is lemon juice or other acidic liquids. Or of course, you can "whiten" enamel using the same oxidizers like peroxides. Keep in mind that enamel is semi-tra