How to clean an old one piece fiberglass (?) tub/shower?

This is how I clean my hubbies shower. Easy Off Oven Cleaner in the blue can. His shower can look bad and this has worked every time. Just be sure to close the door and turn on your exhaust fan.

https://www.hunker.com/13420724/how-to-clean-showers-with-easy-off-oven-cleaner

Yes, that is what I am living in too. In my case a mid-80's box. Vinegar and Barkeepers Friend is what I have always used . See if these help you:

https://homeguides.sfgate.com/cleaning-white-fiberglass-shower-90466.html

https://www.hunker.com/12126351/how-to-clean-and-restore-shine-to-a-fiberglass-tub

https://www.sunrisespecialty.com/how-to-clean-fiberglass-shower

Does your bathroom receive a lot of unfiltered natural light? If so that might contribute to the color change.

Hello, here's some info I located on the web. Maybe it willl help you with your dicolored shower unit.

https://www.hunker.com/13421638/how-to-whiten-plastic-or-fiberglass-tubs-showers

1. Pour undiluted white vinegar into a clean, empty spray bottle.
2. Spray all of your tub's surfaces, including cracks, crevices and hardware. ...
3. Wet a cleaning sponge with vinegar.
4. Sprinkle baking soda onto the tub's surfaces and use the wet sponge to scrub it in gently. ...
5. Rinse the tub well to remove any remaining baking soda.

Easy Off No Fume Oven Cleaner in the BLUE can. Works on enamel, porcelain, and fiberglass

Old plastic/resin/fiberglass as it ages turns yellowish. Only thing I know that works is Gel Gloss and then use a carnuba wax product when clean to protect it.... https://www.google.com/search?client=opera&q=Gel+Gloss+fiberglass+cleaner+whitener&sourceid=opera&ie=UTF-8&oe=UTF-8 They have it at True Value Hardware store, Walmart and Amazon.

you can paint it but once they turn color you can't get that off - mine is 30 yrs old and did the same thing it is light almond now instead of white.

Here are some methods to help you. https://www.sunrisespecialty.com/how-to-clean-fiberglass-shower

Zep Commercial Shower Tub & Tile cleaner to scrub. Then wet & forget shower applied to weekly to keep it clean without scrubbing.

You should be able to paint it. The trick is doing the proper prep work and using the right paint. I'm pretty sure Rustoleum makes a product for this.