Any white letter will get dark over time...that''s why I don't like them...turns into another thing that requires attention. With that said I do have white letter Coopers on my truck(long story as to why I didn't mount them in...still regretting that decision) the best I have found so far is a Scotchbrite pad in some car wash soap. Cleans them up great but they will darken by the next wash time.