Each color line's hair color is different. What may be a level 5 on one is different from another.
What you should do is look at there color wheel and see what is considered a level 5 and take the swatch to your hair to compare it. In order to achieve the same color as your regrowth, you would have to use a filler before you color your hair. The filler is missing contributing pigment for your hair color. Most of the time it is red. You would need to find out the shade of red you need to add to your hair. It is better if you have this done professionally.