Shower pans can become eyesores, even after you clean them every possible way. You may think your only option is to replace the shower pan, but before you do, try painting it. It is a common practice that, if done correctly, can look nice and last a long time.
Clean your shower pan with a sponge and trisodium phosphate cleaning agent (TSP), available at most home improvement stores. Make sure your bathroom is well ventilated and you follow the manufacturer's directions before using. You can also use Mr. Clean Magic Eraser to clean in the grooves of a textured shower pan. Use a fine steel wool to roughen up your surface if your paint directions call for you to do this. Be sure to clean well when done.
Remove the caulk line between the shower pan and the tile. This is probably too dirty to clean, and it will make your project look nicer if you replace the caulk. Paint will not adhere to caulk, and it will be easier to use tape after if it is removed. Use a flat-head screwdriver or utility knife to carefully remove the caulk.
Use wide painters tape to tape around the top rim of your shower pan, and hang plastic drop cloths. Be sure the entire shower stall, except for the pan, is well covered.
Evenly spray the shower pan with a can of epoxy paint. You can also purchase paint specifically for fiberglass tubs and showers. Follow the manufacturer's directions for use, and protect yourself by wearing safety goggles, respiratory mask, rubber gloves and protective clothing. You may wish to practice on a piece of cardboard first to learn how to apply an even coat.
Use fine steel wool to lightly roughen up the paint and remove any drips between coats. Clean the shower pan well before applying the second coat.
Grout after 24 hours. Remove the tape and drop cloths, and carefully apply white grout that comes in a tube. Follow recommended wait times for the paint and grout before using the shower again.