In what is certainly an appropriate, if surprising, twist, sun-yellowing of white clothing can actually be fixed using--you guessed it--the sun as a cleaning agent. Of course, there is a little more to it than that, but if you have noticed that your whites (particularly cotton blends) are looking a little yellowed after drying outside, the odds are that you just need to add a single step to your washing and drying routine to resolve the problem entirely.
Wash the clothing as usual. You might want to use a detergent that "whitens whites" and "brightens brights," but there is no need to bleach the clothing unless the yellowing is extremely severe.
Rinse the yellowed clothing in a lemon-water mix. Place 4 to 6 cups of hot water in the plastic tub. Then add a cup of lemon juice. You can multiply these amounts to create larger volumes of lemon rinse. Be sure that the clothing is saturated with the lemon rinse before you wring it out and move on.
Hang the clothes out to dry. Make sure that they are in full sun. The lemon juice will interact with the sunlight to whiten the yellowed spots on your clothing.
Spritz the clothing with the lemon rinse. If you have the weather for it, you may want to give the clothing an additional treatment with a spray bottle full of the lemon rinse. You do not need to re-saturate the clothes, but you should mist them thoroughly on all sides.
Leave the clothing outside until it has dried completely. If this does not happen before the sun goes down, you can bring the clothes inside and finish them off in the dryer.