Summarize this article:

Testing out a nude before you take it home is essential, but you don't want to apply it to your lips (too many germs!). A good place to try it out is the back of your hand, where you can see how it looks against your skin before you purchase it.  You can also try it on your inner wrist, which may match the color of your face and lips more closely. At some department stores where the testers are closely monitored, an associate may be willing to scrape off the top level of the tester for you. Then, use an applicator to try the color on your lips. Never use the tester lipstick directly on your mouth. Often, the outer edges of your lips will be slightly darker than the rest of your lips, particularly if you have a darker skin tone. When looking at the color, make sure you match the outer edge of your lips. Otherwise, it can leave your lips looking washed out. When picking out a color, think about your natural lip color. Try to pick something that is similar. You want it to work with your lip color rather than against it, as it will be more difficult to achieve a nude effect with a lipstick that works against your natural color.  That is, if you have naturally rosy lips, try to pick a nude with a hint of rose. If your lips have brown tint, find a similar tone.

Summary:
Test it on your hand. Match the color to the outer edge of your lips. Stay in the same shade range as your natural lip color.