The rules
http://www.ncanewfs.org/working/water/pdfs/WTRegs2-2-12.pdf state:
"An underwater retrieve article that is no more than twelve (12) inches long and
four (4) inches tall. When submerged, it should extend no more than four (4)
inches from the bottom and should be weighted evenly so that it will sink
immediately. The underwater retrieve article may be painted or taped to increase
its visibility. Handers may provide a second underwater retrieve article but it must
be identical to the first article except in color. It may be used if the first article is
lost in the water but if a second article is used then the dog must retrieve the
second article."
It does say "may be painted or taped to increase it's visibility"