When buying foundation, do you choose based on your face tone or your neck tone?

You know, your neck and face aren't usually the exact same color. My neck has a bright yellow tone, but my face is red and dull, so I prefer a yellow-based foundation that is slightly lighter than my face to match my neck. When I test products, I apply a little bit to my chin and buy the one that doesn't create a harsh line with my neck.

Where do others set their standards when buying?

Do you buy foundation that is exactly the same color as your face, or do you buy one that matches your neck color so there is no difference, even if it is slightly different?