The standards do not permit you to make a helmet that degrades from sweat, and the EPS, EPP or EPU foam is remarkably unaffected by salt water. Your helmet will get a terminal case of grunge before it dies of sweat. UV can affect the strength of the shell material, though. Manufacturers put UV inhibitors in the plastic for their shells that control UV degradation. If your helmet is fading, maybe the UV inhibitors are failing, so you might consider replacing it.
Chances are it has seen an awful lot of sun to have thathappen.
Otherwise, try another brand next time and let us know what brand faded on you. That is highly unlikely, unless the EPS is placed in an oven for some period of time and baked. The interior of your car, for example, will not do that, based on helmets we have seen and at least one lab crash test of a helmet always kept in a car in Virginia over many summers. EPS is a long-lived material little affected by normal enviornmental factors. In sum, we don't find the case for replacing a helmet that meets the ASTM or Snell standards that compelling if the helmet is still in good shape and fits you well.