Large boots, heavy skiers and long skis = blue springs.
Small boots, light skiers and short skis = green springs.
That said, different skiers have different style and different likings.
If you are in doubt, we recommend to start with green springs.
We have heard that some think you are a better skier if yo use harder springs. In our mind, green or blue springs have nothing to do with skier skills
(note that NTN bindings not are DIN certified release bindings)