Stains on fingers can come from many sources, although the usually stain is caused by smoking. If you are looking to diminish the yellow on your fingers, try this.
Pour vinegar into a bowl.
You can either dip your fingers into the bowl and leave them there or dip a cotton ball into the vinegar and then place on your finger. Leave on for a minimum of 30 seconds (most likely you'll need to keep it on longer for it to be more effective). You can also rub the vinegar into/around your skin if you like.
Remove from vinegar and wipe with a towel. You may wish to wash your hands afterwards to get rid of the vinegar smell, which can be quite strong.