The resins used in gelcoat tend to oxidise with time and solar radiation, forming a white, rather opaque, rather than clear, surface. Although the pigments in literally all gelcoats are metal oxides and are un-affected by the sun or aging process, the white resin oxides will tend to give the appearance of fading. On the other hand the white pigments (usually titanium oxide) will mask the oxidized resins--thus the white gelcoat wille lose its luster(shine) but not display the appearance of fading.