The green hat is for creative thinking. The person who puts on the green hat is going to use the idioms of creative thinking. Those around are required to treat the output as a creative output. Ideally both thinker and listener should be wearing green hat.
The green color symbolizes fertility , growth and the value of seeds.
The search for alternatives is a fundamental aspect of green hat thinking. There is a need to go beyond the known and the obvious and the satisfactory