Which developer level should be used with paul mitchell hair color product?

It depends on your natural color and the desired color. And a lot of other variables.

Are you lightening your hair, covering grey or just going for a different color. Standard strength is 20 volume. Higher lift colors often require 30 or 40 volume.

You should get a consultation with the establishment that sold you the haircolor. if you have colored your hair in the past and want to go lighter, you will need to remove the old haircolor first, usually with bleach. Haircolor will not lighten the artificial color molecules in your hair, only will lighten your natural color.

If this is the case, I highly recommend going to a salon for your color.

Good luck.