Lichen colors vary widely due to presence or absence of special pigments, though in the absence of these, lichens are generally gray or greenish gray. Lichen pigments are pigments or dyes prepared from certain lichens. Many L.p.(eg. litmus and orcein) are derivatives of orcinol, which occurs free and combined in many lichens of the genera Roccella and Leconora. Lichen pigment, which determines the organism's color, also plays a protective role by shielding the lichen from too much ultraviolet light and keeping it from becoming desiccated. The most deeply colored lichens—the yellow, orange, and brown species—are often found in dry, exposed habitats. When a lichen is wet, the cortex becomes more transparent and the underlying photobiont layer becomes visible, making a gray or brown lichen turn bright green or olive in color.  But a variety of bright pigments may be deposited in the cortex: most widespread is pale yellow usnic acid, but other pigments generate a variety of yellow, orange, or red lichens.