Shades for Mac applies a tint of your choice on top of your display to boost your privacy. It can be used either through an optional slider situated on your desktop and in the app's menu, or through keyboard shortcuts. Unlike the built-in brightness controls on your Mac, this app will not save you battery life since it doesn't affect the display's backlighting.
After installation, Shades for Mac resides in your System Preferences as a pane and runs continuously in the background. When the service is started, you will notice a new menu bar icon and a desktop slider that is used to change the brightness of your display. Adjusting the tint color is done via a default OS X color wheel. The utility also provides you with customizable keyboard shortcuts for adjusting the brightness level as well as an emergency shortcut in case the screen becomes one solid color. Another safety feature is the ability to set a minimum brightness threshold. You may also opt to have the app's icon show you a percentage along with its slider. Overall, the app performs well and also proves to be useful for matching the temperature of your display with your surroundings.
If you're concerned about your privacy and want to make it harder for others to read your screen, you should try Shades for Mac. It's a basic app that works as advertised, performing its function properly. Don't expect it to preserve your battery life, though.