BF Goodrich TA Radial white lettering

I have BF Goodrich TA Radial tires on the front of my Duster and while washing the car today I noticed that the lettering was turning yellowish . I tried washing them with scrub brush and car wash soap but no change . The tires on the rear are Micky Thompson Indy Profile and the white lettering still looks good and these tires are older than the TA Radials . Anyone have any suggestions on how to reverse this 'yellowing of the white raised lettering' ? Please advise and thanks .

Terry