Bought car with rear light assembly slightly different from orig. Now rear lamp/stop lens clear, not red like other side. So night light is white light, not red. My first thought to paint interior of lens with red nail polish, two coats. Other ideas like 'red film', red lamp itself, replace lamps with new red LEDs, etc. Any ideas or best method? Cost control v. important.

Not being funny , but are you sure you are looking at the brake lights , not the reverse / fog lights.

Cost control ,what is it a Toyota or a BMW if the former may be able to buy new pattern ones cheaply.

I spray the bulb with red spray paint.....

Yes you can use nail polish to restore lens colors,it was a old trick to pass the MOT test in the UK. Not sure if you could change white to red though. They also used to make coloured tape to repair/change the lens colour. Or use colored bulbs

You can buy translucent red paint, make sure it is for plastics though. If you can't find any go to central or the like to the model/toy section and look for the Tamiya/model translucent red paint.

Other suggestions as someone posted buy a new one it shouldn't be expensive or go to a 'plastics' shop they should be able to vacuum form a new lens for you if removable.