If you are unsure of your shade of foundation (because you have to consider things like skin tone as well as undertone), google "What is my foundation shade"

On the Maybelline Fit me website and others, they ask you questions with pics of models with different skin tones and how your skin acts so that you can gauge.

I think it is a useful, fun and insightful function if you are just starting to think of using foundation. It is a good starting point but I don't think it should substitute having samples tried and speaking to a consultant