Je ne comprend pas du tout cette solution.

The 'this' keyword is only needed in 2 cases: 1) when there is a class field and a local variable that have the same exact name, creating a conflict in the names (which one do you mean when you use the name?):

class myClass {
   int temp = 0;  // this is a class field named 'temp'

   // in parentheses is a local variable named 'temp'
   public myClass(int temp) {
      this.temp = temp; // when you use 'this' the computer now knows
      // that you are referring to the class field. Without 'this' it thinks you
      // mean the local variable
   }

   // in parentheses is a local variable named 'other'
   public myClass(int other) {
      temp = other; // don't need 'this' here because the names don't conflict
   }

and 2) when referring to the current object, which is an advanced topic at this point which you will learn about later and I won't go into more detail here. To make the Circle class valid all the fields that are part of the class need to be initialized. The Circle class has one class field that is a Color. As you say: the constructors job is to make the current object valid, which is correct. So, it has to create a new Color to properly initialize that field of the Circle and make it valid a valid object.