Towels can become discoloured and dingy over time, even with normal use and especially with heavy use or cleaning duties. You can get them looking whiter or brighter by using bleach to restore their original colour. Forms of bleach can be used on colours and whites; chlorine bleach removes stains and discolouration and kills bacteria, while oxygen bleach (sometimes known as all-fabric or colour-safe) brightens colours, but won't disinfect. Oxygen bleach works best in hot water however, which also kills bacteria.
Read the care labels on your towels. If the label indicates "No Chlorine Bleach," don't bleach it. Coloured towels and some white towels should not be bleached with chlorine due to dyes and finishes used, but an all-fabric bleach can be used.
Put straight liquid detergent or pre-wash spot treatment on any stains and rub it in with your fingertips to pretreat spots.
Let the detergent sit on the stain for up to 15 minutes. Do not let the detergent dry on the towel.
Add the appropriate type of bleach to your washing machine. Use regular bleach on white towels. Use non-chlorine, all-fabric or colour-safe bleach on coloured towels. Generally speaking, use 1 cup of bleach for a top-loading washing machine and 1/2-cup for a front-loading machine, but read and follow all product label instructions.
Add your washing powder and towels and launder as usual.
Never put bleach directly on fabric. Over-bleaching weakens fibres.