You didn't get me right.
When you know that a button is going to change the color sometime soon you already have your finger slightly pressing on the mousebutton just expecting that you will have to react soon. That means you know the measure you have to take before the incident. Then you'll react in about 0,2 seconds.
On the other hand, when you don't expect the button to change the color (or don't expect to see a car standing on the track on front of you) your reaction time is usually about 1 second.
The first time is important for Counter-Strike-players, because they expect an enemy to jump around the corner and just have to click at the right time. For driving the second time is more important, because you still have to make up a decision (drive left, drive right, brake, ignore) before you can actually react.
Vain