Looks like they just use Hex # 242424, or rgb- 36, 36, 36 for their color. For anything you want hidden in the background, in this case the ship. (That will only work for this off gray in FJ. Any other website it would just be whatever background color used.) For the image that will disappear, on a different layer use slightly lighter gray Hex #323232, or rgb- 50,50,50 only when you're finished save it to about 75% opacity. Delete background layer and save as a .png.