I do a fair amount of car modeling . I use a pencil eraser.
give the eraser end a round shape than dab it onto a thin layer of paint.
the eraser will absorb just the right amount and get a little sticky
than gently start dabbing over the lettering , you can increase or decrease the amount of white by
dabbing the eraser on a sheet of paper.
I use acrylic paint so if I make a mistake a wet brush will flood it out of the crevices
I finish with a seal of dullcote or satin clear
a lot of guys use the white gel pens , I haven't tried them yet
there are decals also , you have to spray gloss(or future) the tire first, apply the decal than spray a flat or satin to blend them in