Hi Debinmi,
If you compare the light band to the extra heavy band, then yes there is a big difference, but not when you compare the light band to the medium band. Don't get me wrong, there is certainly a difference between all band tensions, but the jump for each different band is not what I would say is huge when comparing saying a light band to a medium band or a heavy band to an extra heavy band. But this will also depend on the exercise and the range of motion of the exercise which means sometimes you can easily use a heavier tension band and other times you won't be able to.
And yes the green band (medium tension)is our "goldilocks" band that most people use and what we use most often in our workouts. So, this is a band you should certainly have.