Check out Pixowoo
's video on concealing tattoos. The key is to pick complementary colors to erase pigments on the skin. If your tattoo is red, use a green concealer first to pull out those warm tones, and then a "skin tone" concealer over that, and powder to seal it in. I believe Kat Von D has a tattoo concealer that you can buy at Sephora, but it's probably a lot more expensive than buying a concealer wheel and matching colors.