The method I used gave great results. First scrub them with those steel wool pads that have the soap in them. You know the ones you get at Woolies to scrub the fry pan. Then get some Stanley knife blades. Using care scrape the blade over the plastic carefully removing the discoloured top layer. You need to be careful to not let the edge of the blade dig in or gouge the plastic. You will quickly get back to the original colour. I then used a plastic bumper restorer liquid on them. It was like super thick armour all. They came up like new and like all red plastic on 80's Hondas, they were nearly completely white.
If they are only dirty, use a scouring pad and Jiff.
I think paint is great but as soon as you scratch it the old plastic will show through. You can't paint a plastic fuel tank either as the vapour will lift it straight off.
There are a million YouTube videos showing plastic restoration. A lot suggest heating the plastic to bring the colour back. I wouldn't. It makes the plastic super hard and you can't scrape it then.