Wiktionary
vb. (past participle of gray out English).
Wikipedia
A grayed out graphical control element is one that is displayed with a light shade of gray (or lighter color), to indicate that it is currently disabled, and cannot be operated nor selected by the user.
This convention is widely used in graphical user interfaces, to visually indicate which actions cannot be taken in the current situation. This approach makes it possible to disable a control when it cannot be used, and give a clear signal of that state, while maintaining the graphical control element in a consistent location, thereby minimizing confusion for the user. A graphical control element may also be shown grayed out in its enabled state, indicating that the user cannot disable it.
Usage examples of "grayed out".
Across the street, she could see bits of the downtown skyline through gaps between a couple of the houses and a squat, brick machine-shop, but most of it was grayed out into dim, jerky geometric shapes in the smog.
He must have grayed out then for several seconds because when consciousness returned, he was once again lying flat on his back.
The LCD screen filled the small lightless room with soft blue illumination that grayed out all the color of his BDUs and the blue steel of his Colt.
At his feet, Rakshal writhed and twisted, blackening into a blasted, lifeless thing until Heyoka's vision grayed out and he had to admit he had nothing more.