The most permanent way is to paint them with grey/black bumper paint (found at any automotive store, Pep Boys, A/Zone NAPA, etc)
The tedious part is the masking, actual painting should not take you more than 15 minutes.
Done properly it lasts for years
2000 V70R Black, 121,000 miles (Wife's)
2000 V70R Silver, 36,200 miles!!! SOLD (I will regret this)!
2007 XC70 White, 122,000 miles, loaded and mint, mint, mint in all aspects,
Contact info for purchases: jrl1194 (at) aol (dot) com