Level 3 hair color products are permanent, though the untreated roots will eventually need new color.  To give re-growth the same color as the rest of your hair, follow these steps:
Mix both product components.  First apply the color along the part line.  Divide strands using the tip of the application bottle or with a hair color brush.
Also try the Poly Color applicator.  It enables easy and targeted application of hair color.  
The duration of coloring for the roots is 20-30 minutes.  Thereafter, apply the color all over hair.  Leave in for another 10 minutes.  Then rinse out the product and wash as usual.