The majority of sub <£10 trim gunks will invariably be a complete waste of money and will often only last until the next wash. The same effect can achieved with WD40.
A heat gun will give a longish term temporary fix.
Gtechniq C4 is good stuff, though again not truly permanent. 6 months to a year for this before renewing. Its not cheap at around £25. A little does go a long way, but its does have a finite lifespan once opened allegedly, so don't leave it hanging around for too long.
I have used the plasticare trim dye, but for me it was hopeless. I have since read that it can be a bit hit or miss depending on the plastics involved.
If you want to dye your plastics I would give the plastics a thoroughly good degreasing, mask up, and go for something like VHT Vinyl Dye.