Inside Apple ▸ System Preferences under Universal Access, you can set your display to grayscale mode. But you cannot find any keystrokes to trigger grayscale display.

In my opinion, grayscale display is more important than good for nothing white-on-black display. You will need to make your display grayscale to your needs, such as printing files or designing web that can also be used by color-blind society.

I wrote a simple AppleScript Application and will share it with you. Here is what it looks like:

Download Display Grayscale Application [AppleScript]

So with a single-click, you can have your Mac automatically open System Preferences, go to Universal Access, move to Use grayscale checkbox and toggle it for you. Though the processes are not instantly done like when you use keystrokes, but at least you don't need to open System Preferences by yourself.