The green paint is just to identify the round as M855. Other countries may have different markings.
An unpainted projectile could come from another country, a production overrun domestically, or the bullets could have been tumble shined after being pulled. Sometimes you just don't know.
In the back of my mind there is a nagging feeling that there was some M249 linked ammunition that had the M855 bullets unpainted, only the tracers painted. But I can't seem to recall where I saw that. It has been a couple of years since I shot a SAW so forgive me a failing memory.
Machine guns are awesome until you have to carry one.