iPhone Compass App

On the iPhone, when you use the Compass App’s horizontal level feature, it provides a visual feedback at zero degrees by changing the background from black to green. Similar visual feedback might be seen at CVS or Walgreens when the LED on barcode scanner turns green from red after scanning an item.

Color change is good for an objective YES/NO feedback but when it comes to gradations, studies have shown that humans are more sensitive to changes in audio pitch/frequency change than gradually changing color gradients.