The fourth (yellow), is for touchscreen calibration.
The touch screen offers access to features like calibration options, chronograph and a thermometer.
Not only does this bad boy have an alarm, a calender, a stopwatch, and world time for 30 cities, but it's also solar powered and receives time calibration radio signals to make sure the time is always accurate.
However, since this is an area that may require delicate calibration -- especially if you're on prescription meds -- you should always consult with your doctor before embarking on self-medication with chromium.
By using hexadecimal notation, this can be exact regardless of the screen calibration or even color blindness of the people working on it.