In the United States:
A law requiring that a "toy, look-alike, or imitation firearm shall have as an integral part, permanently affixed, a blaze orange plug inserted in the barrel of such toy, look-alike, or imitation firearm" was passed as Section Four of the Federal Energy Management Improvement Act of 1988 (PL 100-615). The law provided for alternative markings if the orange plug could not be used.
Makes it unlawful to manufacture, enter into commerce, ship, transfer, or receive any toy or imitation firearm which does not have a marking approved by the Secretary of Commerce. Requires each such toy or imitation firearm to have a blaze orange plug inserted in its barrel as a permanently fixed and integral part. Provides for alternate marking and exceptions in certain instances. -Federal Energy Management Improvement Act of 1988 (Library of Congress)
Sources:
http://www.libraryindex.com/pages/1738/Firearm-Laws-Regulations-Ordinances-TOY-GUNS.html
http://thomas.loc.gov/cgi-bin/bdquery/z?d100:SN01382
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It is a Federal offense to be in public with a look alike firearm, without any markings designating it as such. I personally use orange reflectives whenever I go public with my Nerf props or otherwise.