When choosing a concealer, you should consider both consistency and color. If you are covering major undereye circles, you're going to want a concealer that provides major coverage. Test the concealers constancy by dabbing it on the blue veins on the inside of your wrist.
Yellow-toned products are your best bet for covering up undereye circles. Orange-based tones blend best into dark skin. You want a shade or two lighter than your foundation (remember, your foundation should match your skin almost exactly). Avoid greasy or chalky formulas that are too pink or white.
Test the color on the area of your neck below your ear. The color should be slightly lighter than your foundation. If you go too light, you risk looking like a raccoon. Just like you will foundation, you may need a lighter concealer shade in winter than you would in summer.

