why do your fingers trn yellow when you smoke?

It is the nicotine in cigarettes that stains the fingers and teeth yellow. Nicotine is absorbed through the skin and the mucosal lining of the mouth and nose. It is also absorbed when you inhale smoke into your lungs.

http://www.hc-sc.gc.ca/hl-vs/tobac-tabac/quit-cesser/fact-fait/index_e.html