If your tyre pressures are between 26-29psi (1.8-2.0bar) you need to see mostly green with a slight band of red... Like the second cap from the left above  
 
If your tyre pressures are between 30-34psi (2.1-2.3bar) you need to see all green... Like the third cap from the left above (this is 80% of the cars on the road)
 
If your tyre pressures are between 35-44psi (2.4-3.0bar) you need to see mostly green with a slight band of amber... Like the fourth cap from the left above

If you see equal amounts of green and red this indicates severe under inflation 20-22psi (1.4-1.5bar)... Like the first cap from the left above

Should your normal reading change then you will know you have lost tyre pressure and you should re-inflate to the correct level.