Query: What color were cartridges produced by Nintendo for internal use?
Context: 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.