Trying to identify a novel I read years ago (probably in the '80s) in which the protagonist (late teens, early 20s?) is gradually forgotten by and ceases to be able to interact with the rest of the world (all of which gradually fades to gray).
In the beginning, as he is slowly fading out (not realizing it) he goes down to breakfast with his parents, and his mother has only set out two plates (one for his father and one for herself) — she'd forgotten to set one out for him. He reminds her. Foreshadowing, eh? But gradually everyone forgets about him and ceases to be able to hear or see him, and fade to gray.
Some other things apparently similarly get "forgotten" however — e.g., occasionally he finds colored, not gray, cans of food in grocery stores, which he can interact with and eat — so he survives. He then meets a girl who has similarly been forgotten by the world, and they spend time together and fall in love.
At the end,
he is gradually starting to fade back into the world (she isn't) and she puts writes a note about them and puts it in his pocket, in the hope it will fade back into the world with him. It does, and he sees it one day — but has no idea what it is or any memory of her. Sad.