To invert the colors on your mac, use the shortcut ⌘ (command key) + option/alt + control + 8 (all at the same time). For those not confortable with shortcuts, you can do the same by going to (Apple logo at upper-left hand corner_ -> System Preferences, and clicking “Universal Access” under Personal. From there, checking ‘White on Black’ will have the same effect.
Take a look at the two images below of what my screen normally looks like, and what my screen looks like when the colors are inverted
Do keep in mind if you take any screenshots in the ‘inverted color’ mode, the screenshot will not come out with inverted colors. If you would like to have inverted colored screenshots, you must manually invert the image in an image editing program.