I think Happygirl was saying that the hairdresser mixes up the colors. If the OP could just buy Aveda brown #23, there would be no need for the hairdresser.
I think most people think hair color mixing is similar to fingerpaints. You mix red & blue together and get a purple. Add white, you get a light lavender. That's it.
When I took my BFF, who had the Jello green hair, to Vidal Sassoon, the colorist knew immediately what happened. They explained it to her. She had bleached her hair blonde so many times, that the process stripped away any RED color in her hair.
Her cheapo, inexperienced hairdresser didn't know you can't go directly from bleached blonde back to brown in one process. Since her hair was missing natural red, when the brown haircolor mixed with her hair, it had no red to grab onto. The opposite of red is GREEN, and that tint is what ended up in her hair. It was still blondish with a Jello green tint, like she had rinsed her hair in green Jello.
At Vidal Sassoon, the colorist told her they had to do two steps on her. The had to color her hair bright red first, red like Ronald McDonald red.

he said it had to be that color first as, when he later added the brown dye on, it would grab onto the red and mix together properly, creating a wonderfully natural auburn red brown, matching her real hair color.
While I never had my hair colored, that certainly made me appreciate that haircoloring, in the right experienced hands is an art. He had to know
which color red and which mix of brown to mix together in 2 steps to get the right final color.