Why the squares are colored the way they are? Shouldn't the first square be red?
This is a popular question and of course we understand why. The red color is one that best attracts people's attention. It is associated with action and activity. It is usually used to warn about danger (traffic 'STOP' sign), so people are used to paying more attention to it.
Thus your first wish is to put every action into the first square - the Urgent / Important one. You see all your actions both Urgent and Important at the same time. And you rush to do them all. This is the view of classical todo lists. But, it is not if you think about planning.
The first square should hold only the actions, that are burning right now, the crisis actions. That square should either be clear or you be running doing the action from it. The actions from it are usually stressful ones and you shouldn't have them in your normal, everyday life.
The more important among the squares, or even the most important one, is the second square.
Here you put your Important and Not Urgent actions. Don't be fooled by the 'non-urgency' of the actions - the fact that they are not urgent doesn't make them less or not important. It just means that they are under your control - you know about them, you know how important they are and you have thought and planned when you will do them. There's no need to throw everything you are doing and run to do them. So, this is the most important square with actions for you. And since it is the most important, you should be attracted by it more than by others. Thus the red color of the square.
The other colors were chosen so that they try to distract you less.