iOS Making background go gray on button press

[Samppaa]

Hey! What I am really asking is that how do I make the background go more "gray" when I press a button. When I press the button it brings up a picker wheel, I want to tell the user that the picker wheel is now the active component by making all other things more gray if you know what I mean. I got a tableview, some buttons and a navigationbar which I want to make more gray, like it shows they aren't active. Is there anyway to put a layer over them or something? Sorry if I explained this very badly, just ask and I'll show a picture what I mean or something. Thank you.