It probably depends on whether the lettering is just plain black rubber, or is moulded into the tyre in white rubber.
I have a set of BF Goodrich Comp T/A's on my old 'Vette with moulded-in white lettering & I found the best way to bring it out was to scrub the lettering with a pumice stone & an abrasive kitchen cleaning paste - comes up like new.
If your lettering is plain black, you might like to try a "vinyl dye" from VHT - it is normally in a spray-pack, but you could spray a bit in the lid, then brush it on.
I have used the VHT dye on the 40's vinyl-covered dash & f/glass demister panel (black), the 'Vette's door panels (blue) & have found it far superior to any of the "surface paint" options. The dye seems to soak right into the material. It should work ok on rubber (although I haven't tried white as yet). Still, it is fairly cheap & might be worth a try.
Kind Regards,
Peter D.