Here's a video that shows painting on your own custom white letters with masks that you can buy. The madness starts at 5.30, and goes to 15.15, enjoy.
Hoosier paints their name on all their race tires so it has been done for decades.
https://www.youtube.com/watch?v=uFGlGzRvOJM&t=8s
Myself, I hate white letter tires, but if you like them, here's a way you can have any tire converted to white letters.