Nails are like teeth: unless you're doing something to make them yellow (something occupation-related?) they may not be naturally very white. But if the cosmetic aspect really bothers you, go to a drugstore and look in the section for nail care. Most will carry a white pencil which is meant to run under the tips of nails to whiten them.
2015 Connor spruce/Indian rosewood
1978 Ramirez 1a cedar