The safest bet is to look for a shade that has the same depth and undertones as your foundation. That will allow you to use the concealer wherever it's needed. If you are only covering blemishes, stick with that method. If you are covering dark circles or redness, there are other things you can do. You can choose a shade that's about one shade lighter than your foundation color for dark circles. You can even go slightly pink or peach compared to your shade (don't go too far off). To cover redness, you can try a green color-correcting concealer topped with a foundation-matching concealer.