I have used the paint pens from Walmart in the craft section with good results, just clean it well first, and your not pinned down to staying with one color, my next ones will be green lettering.
+1 I work in the RC industry and we love using paint pens for all our rapid prototyping parts and tire identification markings. Nothing really special about them but they get the job done and are cheap if you need to do any touch-up work.
I remember "back in the day" using any old white paint laying around and none of it would actually dried on the rubber letters - was always sticky
Still make it - and I have used them in the past and they do the job ...... Ebay ... under $5.00 http://www.ebay.com/itm/2-Qty-Marka...ools&hash=item19d829ae44&vxp=mtr#ht_247wt_721