Concealer is used to disguise blemishes not fully covered by foundation due to its thicker formulation. Color corrective concealer is applied before foundation in small areas using shades like green to reduce redness or lilac for a healthy glow. Skin colored concealer is the same or one shade lighter than the skin and is applied after foundation. Dark circles, spots, and blemishes can be concealed using color corrective or skin colored concealer applied with a small brush and blended outwards from the area.