CONCEALERS can dramatically change your appearance. Used correctly, concealer can brighten a tired face, cover up blemishes and it can totally conceal under-eye circles. In fact, many beauty editors use concealer primarily and skip foundation completely. A basic rule of thumb is to choose a concealer that's yellow-based and a shade or 2 lighter than your skin tone. If your skin is darker in summer, you should choose a darker shade for summer months and a lighter one for other months.