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.
That Nassau Co. thing has me wondering about all those guns, especially in recent years, that have been marketed to women with pink frames and such. That law would seem to put them in violation and I'd be very curious to find out from any Nassau Co. resident members whether or not those guns are sold in local stores there.