That's what I thought you might be referring to. There is a whitewall under white letter tires. If you have ever seen a badly scuffed white letter tire you will see this. Try using some black tire paint/dressing and a fine artist paintbrush. In the future don't be so rough in cleaning letters.
Another safer way to get white letter tires is to use a white tire crayon and go over letters of a Blackwall tire. Works best if they are raised solid letters not just outlined. It is a lot of work, but most people just use white letter tires on old muscle (show) cars.