Dyes are chemical compounds that are added to fibers to give them color. Sometimes these dyes react with chemicals or gases and changes in the color occur.

Fume fading is a reaction to gaseous pollutants, such as oxides of nitrogen or sulfur, in the air. It is a gradual change, accelerated by sunlight, heat, high humidity and the presence of acid on the fiber. The most common color changes are blues to pink, greens to yellow, and browns to red. The color change usually starts at the tips of the tufts and progresses toward the backing.

Ozone fading is caused by ozone gas in the atmosphere. It is also accelera1ed by high humidity and heat. Ozone is more prevalent around electrical motors, fluorescent lights and during lightning storms It is also formed by a reaction between light and pollutants in the air. Fibers subjected to ozone fading may lighten, turn white, or change from one color to another as in fume fading.

Some carpet fibers are dyed with Indicator Dyes. These dyes are sensitive to either acid or alkaline chemicals. An alkaline-sensitive dye will change color if exposed to ammonia or high alkaline (high pH) detergent. The color often can be changed back with dilute acetic acid (white vinegar). An acid sensitive dye will change color when exposed to vinegar or other mild acids (low pH) used in cleaning. The original color often can be restored with dilute ammonia. These color changes may not be permanent and often can be reversed. Other color changes due to strong chemicals (concentrated acids and bases or other reactive chemicals) are not a result of this "indicator effect" and may not be reversible.

Color changes that become apparent after cleaning are sometimes incorrectly blamed on the cleaner or cleaning process. In many cases, however, the color change is due to the ravages of time—the aging of dyes and fibers. Cleaning reveals the true color by removing dirt and loosened dyes.