While adding a bit of shampoo or conditioner to increase the amount, you will only add cleaning products and not the stuff that makes your hair color differently. The result of this is poorer coverage of each hair strand and maybe even a different shade of color.
I'll tell you my experience just to compare:
I asked my hair prof to color my hair in a level 4 almost 2 months ago and when she was almost done covering the hair in coloring cream, she noticed that she didn't have enough. She didn't start adding shampoo or conditioner. No, she used the same products to get that little extra products so she could cover all my hair strands evenly with coloring product. It looked very nice.
BTW,
I'm getting a level 3 coloring at the 24th of August. Can't wait for it, especially since I start to see the difference in color of the roots.
Counting the days until it's 24/08. <img src="/images/graemlins/laugh.gif" alt="" />