If this comes with 20 volume developer you can make it darker or lighter. That's what developer does. 20 volume can lift the hair 2 shades and if you are using a darker shade it will deposit and go darker.
If you are going lighter you must really be careful because hair has underlining pigment and when you lighten the hair 2 shades that underlining pigment may be red, orange or brassy yellow.
Also if you have previous color on the hair, color won't lift color so your grow out will lighten and the rest will stay dark.
This product is really for professionals. I am a hairstylist and I have seen some disasters when people try to color there own hair :clown: especially going lighter! :faint: