There are two causes for dull or dingy carpet:
Carpet soils contain huge amounts of grit. Grit is a microscopic rock with many sharp, jagged cutting edges. Walking on soiled carpeting has a sandpaper-like affect on the fibers. The grit gouges and scratches the yarn’s smooth, polished finish. This sand paper effect can also be caused simply by wearing indoor shoes; whether clean or soiled the soles of the shoes have a very similar effect upon the carpet fibers.
After it has been scratched and cut by abrasive grit; carpet fibers no longer reflect light rays as well. The result is a dull or gray appearance. The damage caused by abrasive wear is permanent and no amount of cleaning can remove it. These scratches are an ideal place for new soil to become lodged. Since soil absorbs light the carpet will appear dirtier in traffic lanes. Further traffic without cleaning leads to soil fusion.
Ground in soil in pathways can be difficult to remove; so never permit pathways to become excessively soiled before you clean.
Additionally the grit also causes the carpet tufts to fall over and thus the start of that packed down appearance. As the carpet packs down the tips of each carpet tuft begins to fray and/or plume. As this worsens the path way begins to appear very much worn and that dull gray shade is readily visible even after cleaning. At some point these areas of the carpet will appear gnarled and worn out.
If you want to keep your carpet looking beautiful, clean and sanitary in your high traffic areas professionally clean your carpets as soon as soil begins to show. Keep in mind that carpet is designed to hide soil from your eyes. As a result your carpet may contain large amounts of gritty soil long before the surface appears soiled.
Your best defense is a planned cleaning program; whether semi or annually professionally cleaning your high use rooms or areas will keep the sticky soils at bay. As a result the gritty soils will not stick to the carpet fibers and prematurely cause the above mentioned damages.