Editors Note: This is a product that is marketed for marine, RV and auto use.
If the plastic is yellow from age, sun or heat exposure , and has become brittle or crumbly, there's not much you can do without chemically melting & reforming the plastic. (you don't want to do this. ever.)
Like the previous poster said, you CAN paint it, but make sure you use a plastic-bonding paint (like Krylon Fusion) or it'll unevenly flake off and look bad. But if the yellowing is only on the surface, and is due to chemicals (aerosol, hair spray, cleaner fumes) smoke, or dirt, you can clean it with bleach & water. Some plastics can be cleaned with isopropyl or denatured alcohol. Spot test first to be sure.
Also, yellowing or clouding on some acrylic or more expensive items can be buffed with a super-fine steel wool or polishing compound to make them clear. Exampels would be those expensive room light covers and car headlight covers. Be careful to spot check, and use the finest abrasive you can find, though, 'cos these things scratch.
First, take them down and clean them really well.
Try soaking in bleach water.
Use some Krylon paint for plastic and spray paint them white again.
When my father in law passed away, a distant family member, who is a firefighter, visited mom's house and said that when the smoke alarms turn yellow, they need to be replaced because they weren't reliable any more.
Add your voice to the conversation. Click here to answer this question.