I've done this using 'Tire Paint markers' from a Harley dealership. The Tire Paint holds up well and was only subject to discoloration from road grime. My past experience with using oil based enamels for the job was that they look great for a while, but then the stiff enamel can't hold up against tire flex and the finish gradually flakes off..
I'll be curious to hear how your job holds up.. Looks good though.
Too bad the 'white letter' tire trend died out and Tire manufs stopped offering them like they used to..