His eyes are brown with flecks of gold that lighten and darken with his moods. It may darken, thicken, lengthen, and/or define the eyelashes. Additionally, the lenses can electronically darken to compensate for brighter or darker environments. Photochromic lenses gradually darken when exposed to ultraviolet light. "Lighten Only" has the opposite action of "Darken Only". However, as the hair grows out, it will darken again to the proper shade. Often, natural blond or red hair will darken to a brown color over time. Pregnancy may cause the labia minora to darken in color. Oiled wood may be yellowish and is likely to darken with age. A well-prepared ink would gradually darken to an intense purplish black. After that, the sky will slowly darken until it reaches night. The cloth had to be dyed several times to darken the color. Oil products tend to darken leather from its natural color. If left untreated, the skin may darken and become leathery and cracked. Particles from this auroral interaction darken the Jovian polar regions at visible wavelengths.