Fun tip for really pale girls: you know that sickly green cast you get in cheap fluorescent lighting? If you still have that when you go outside, you're probably olive!
Now, I don't mean just kinda fair. I mean you'd make Zooey Deschanel look like a tanning fiend if you stood next to her. Since olive is an undertone, like yellow, red, peach, or rose (a sort of bluish-pink) you can find it on ANY depth of skin. Prescriptives custom blend foundations are great at pinpointing your exact shade (they hand mix them for you, and save the recipe for later).
I personally almost never go out in the sun, due to a mixture of lupus-induced photo-sensitivity and the fact that I like sitting on my butt all day reading more than I like interacting with real live humans. As a child though, when I would stay fair yet not have the complexion of a mildly soiled sheet as I do today, my skin would take on the wheat color common to many fair olive skinned gals. Actually, in places like India, the Mediterranean, and Asia, fair olive is usually called wheaten because of it's neutral tone with a hint of gold, just like a wheat stalk. Yes, you may lean a bit warmer or a bit cooler but olive skin is considered a neutral undertone. That said, more girls can get away with yellow-based makeup if neutral or olive is not available. I implore you not to go pink. It will make your face look grey. I think a lot of Asian girls who have had the misfortune of tangling with Cover Girl can attest.
Anyways, if you're really screwed when it comes to finding foundation and the only place you have to shop is Walmart or something just find a brand like Loreal that offers multiple undertones and grab a neutral and a warm in your depth and blend them together in different ratios until you find your balance. You won't need too much of the warm. Some brands also sell pigment to mix with your foundation. Try adding greens and pale golds to get the right blend.