There are a number of variables here but as a rough guide the below information should be useful.
If the goal is to assist while doing an exercise such as pullups, depending on your strength, it could be a very strong band (easier), or a lighter strength band (Harder). So: green band for the easiest assisted pull-ups on our Essentials shop, and then working down to the red band for much harder assisted pull-ups.
If you rather want to use bands to provide resistance and do other exercises, we would always advise to start with a rather thin band. In which case, you should start with the red band, and then go up to the green band for a much harder resistance exercise.
As a rough guide, if you have to do 20 Reps in a Workout, and with the band you are using, you can only do 2-4 reps and then need a break, the band is too heavy. If you can do all 20 reps very easily unbroken, the band might be too easy, and you could consider a heavier band.