Countries that previously issued 5.56mm rifles with a 1:12 barrel twist will mark their SS-109/M855 ammo with (usually) green bullet tips, to prevent the ammo from being accidentally fired in the older 1:12 rifles. Also, countries that regularly supply other countries with older 1:12 rifles usually mark their bullets for the same reason. Countries that didn't adopt 5.56mm rifles until the NATO SS-109 standard was adopted usually don't mark their ammo with green tips, as they don't have any old 1:12 rifles to be concerned with. Note that many other countries that now use 5.56 weapons were still using 7.62mm rifles until recently and never used any other ammunition than the SS-109/M855 and L-110/M856, so they don't mark their bullets with green or orange paint unless they intend to sell it to countries who require these markings (the US, Germany, and Belgium, primarily). They also typically refer to their rounds by the FN bullet name.