Concealer for your dark circles (depending on how dark they are) should be one or two shades lighter than your natural skin tone. Sometimes, people with NC complexions use NW concealers because they have a pink or peach pigment that helps correct the color of the dark circle.
However, concealer to be used on other areas of your face should be the color of your skin tone. If it's to cover a blemish that is too red, some use a green concealer underneath to correct and cancel the red color, however some are of the opinion that using color correctors is basically altering a discoloration with another discoloration. Nevertheless, you'll have to cover that area with a concealer of your own skin tone or blend your foundation by patting it in if it has enough coverage. If you have pink areas like your nose or chin, you can use a yellow concealer to neutralize the pink, but as always you'll have to cover with another concealer or your foundation if the coverage is sufficient.
Here's what I use:
Bobbi Brown's Corrector - For dark circles and then finish it with either Clé de Peau Concealer or Bobbi Brown's Creamy Concealer.
Laura Mercier's Secret Camouflage - For blemishes
Avoid using very emollient or moisturizing concealers meant for the under eye in other areas of the face. While you can use a drier concealer for your under eye area mixed in with some eye cream, if you use a more moisturizing concealer on blemishes you might end up aggravating the blemish even more.
Hope this helps...
