Is there a way to make it very obvious that my VirtualBox window isn't in focus?

The problem I'm having is switching between workspaces on Ubuntu or OS X and then trying to type in my Windows virtual machine, only to find that it's not in focus. The Windows window looks like it's in focus (based on the title bar), but I'm actually typing in Firefox on the host machine, for example. It's even worse because the Windows text insertion cursor is blinking to show focus.

Ideally, I'd like the VM's display to get unsaturated (e.g. a "partial" grayscale) when not in focus, just to prevent this keyboard-focus problem. Other options would be fine too, as long as I don't have to second guess where my focus is.

I'm not using the seamless mode -- the display is all within a window.