My daughter was doing this all the time and our dentist told her that once you get to a certain color - you cannot get any whiter and in fact, your teeth will start to turn BLUE. She stopped doing it right away. ha ha ha
You know what I have found that works... Peroxide. After I brush my teeth I use a cotton swab to put some on my teeth and try and let it set there as long as possible while doing my makeup. Works great and is cheap and easy.
I actually keep my toothbrush soaking in a small cup of peroxide between brushings. I brush with the peroxide first then rinse and floss then I brush with toothpaste. I have very white teeth by heredity, but they had yellowed some with aging, this routine helped bring the white back. Just don't swallow it.
I actually got the hint to do this from my mom years ago. She got it from a dentist. I also had an oral surgeon tell me to rinse with peroxide after the surgery he did on one of my back molars a few years ago.