The Finder window buttons were inspired by traffic light colors.
Back then, even Apple had gray top-corner window buttons. Jobs felt that the plain uniformity of the buttons made it difficult to distinguish what each button does at a glance. He suggested that the buttons should follow the colors of a traffic light, giving users visual clues to their respective functions—red for closing the window, yellow to minimize it, and green to expand it.
The creation of OS X revolutionized UI in its time and continues to influence UX decisions spanning desktop, web, mobile and even wearable interfaces. It wasn’t about turning the existing interface into eye candy; it was about making it simpler for those who are interacting with it—designing OS X for the people, not the system.
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