The purple patches show where the net force is close to zero on an object in the rotating frame of reference of the Earth around the Sun.
Objects in such a state experience three forces: The gravitational attraction of the Sun (S), the one of our planet (p) and the centrifugal force.
At the five Lagrange points the sum of the forces is zero.
Only L4 and L5 are stable, objects in the other points will tend to wander after some time.
To make them visible in this image, the mass of the Earth has been exaggerated by a large factor.