I don't believe there is any such current law in regards to what color you can paint a gun in either the federal or NY state laws, though IIRC such laws have been proposed.
I did however find this specific Nassau County regulation
Nassau County, New York - Miscellaneous Laws (
http://www.nassaucountyny.gov/website/GenericServices/docs/NassauCountyMiscLaws_Dec2008.pdf) - December 31, 2008 (page 172 of PDF file)
TITLE 69 - DECEPTIVELY COLORED HANDGUN LAW
§ 3. Definitions.
b) The term "deceptively colored handgun" shall include any handgun which has a substantial portion of its exterior surface colored any color other than black, brown, grey, silver, steel, nickel or army green. ...
§ 4. Violations.
b) It shall be unlawful for any person to possess a deceptively colored handgun or a deceptive coloring product except as authorized by section 6 of this title or for any person to attempt to possess a deceptively colored handgun or a deceptive coloring product except as authorized by section 6 of this title.