There are several reasons why fabrics greys, and become dingy, including not using the right amount of detergent ( i.e. Using too much or too little detergent), insufficient rinsing, and /or the wash water temperature is too low.

To refurbish clothing from these discolorations:

Wash with a permanent press cycle in hot water, use a cool-down rinse on permanent press and use one cup of water conditioner instead of detergent.

If the discoloration remains, either repeat this procedure or wash with the correct amount of detergent and either all-fabric Oxygen bleach or chlorine bleach, if safe for the fabric. (Always check for colour-fastness first)

If the fabric is white, consider specialty products available in Super market, to whiten the fabric.

Always separate and wash your whites separate from colours –  don't put heavily soil garments with lightly soiled items.