A product called liquid paint.I use white for my 20 yard yellow for 30 and 40 any other color.They make geen,red orange blue&black. I simply put a dot for a marker at the yardages,it works great for me with my Impact archery Cosmic sight. Heres a link to a similar liquid paint pen http://markingpendepot.com/index.asp?PageAction=VIEWPROD&ProdID=16