Soiling
Oily/sticky soils will cling to fibers causing yellowing/browning conditions - especially in high traffic areas. Driveway sealer can be a primary source of this type of soil. Cooking oils, and animal fats are another common oily soil source.

Fiber/Yarn damage
Dry abrasive soils scratch fiber causing dingy, dull appearance even after soil removal. The damage actually changes the way the fibers reflect light. This condition normally occurs in traffic lanes.

Urine
One of the major causes of yellowing/browning on carpet is urine. Often, a newer spot appears yellow and an older spot is brown. In some cases, normal cleaning will remove urine spots. In other cases, proper spot removal techniques and agents will be effective. Older urine spots are difficult to remove entirely as hydrochloric acid in urine alters carpet dye permanently.

Soil filtration
Appears as dark lines around baseboards, under doors and curtains, and around air registers. This condition occurs with central forced air heating or air conditioning systems. Air is drawn around the perimeter of the room to the air return. This air is filtered as it passes over and through carpet tufts. Most of the time, soil filtration lines are permanent, however, they usually can be lightened by cleaning.

Fading
Sunfading: Ultraviolet waves can damage the dye on textile fibers, causing the colors to fade or change. Most fibers are treated with two or more dyes to produce the desired color. One of the dyes may be affected by sunlight more than the other. This would cause a color change. If each dye is affected similarly, the overall color may appear lighter. Lighter colors fade more quickly as there is less dye to produce a color.