Article: The back of the cartridge bears a label with instructions on handling. Production and software revision codes were imprinted as stamps on the back label to correspond with the software version and producer. With the exception of The Legend of Zelda and Zelda II: The Adventure of Link, manufactured in gold-plastic carts, all licensed NTSC and PAL cartridges are a standard shade of gray plastic. Unlicensed carts were produced in black, robin egg blue, and gold, and are all slightly different shapes than standard NES cartridges. Nintendo also produced yellow-plastic carts for internal use at Nintendo Service Centers, although these "test carts" were never made available for purchase. All licensed US cartridges were made by Nintendo, Konami and Acclaim. For promotion of DuckTales: Remastered, Capcom sent 150 limited-edition gold NES cartridges with the original game, featuring the Remastered art as the sticker, to different gaming news agencies. The instruction label on the back included the opening lyric from the show's theme song, "Life is like a hurricane".

Question: What is the standard color of all other cartridges?
gray