As far as I can tell,the best way to do it is to make the tag first (letters etc.) and then put it on the mold where you mold the main part(white rubber part).
For that you'll need a mold to make the tags and you need to re-design the mold of the main piece, for it to an insert for you to place the tag before molding.
Other option is to cut the tags from a sheet of the color material.
Hope it helps!