I'd hafta see your hair to properly evaluate but it sounds like you have two toned hair or what I like to call HOT ROOTS.

Your regrowth needed to be treated differently than your previously colored hair.

How red is it? To neutralize red you need to use a color opposite the color wheel.

As you can see opposite of Red is Green so a green base color would neutralize that red color for you. Now without seeing it I can't properly evaluate it. Many color lines don't have green base colors anymore though so you would need to mix a gold base & blue base (Ash) make sure it's not blue-violet....to make green. You would be toning it here so I wouldn't use anything stronger than 10 volume developer...try it on a test strand for lets say 10 minutes and then rinse and blow it dry and see how the result is....if it's still red you can always put more back on for 10 more minutes and see the result....it depends on how red it is and what you are looking to achieve. Stay within the current level of your haircolor if you used level 6 then use a level 6 with a green base to neutralize it.

Also one other note...you originally used a level 5 color...then applied a level 6 color over it. One of the fundamental rules of color is that color doesn't lift or lighten previously colored hair so that is a no, no. The only thing that lightens already artificially colored hair is bleach.

G'luck

the thing to remember is this once you colored your hair and or bleach your hair it will always without a doubt FADE!!Especially if you want to go back to your natural haircolor. If it has been bleached you will want a tintback.

My hair for example is dark brown level 3. If I colored it say to enhance the brown hair or bleached it to make it lighter, then decide to go back to my (natural hair color) yes I can use a brown hair dye but it will almost always fade to a reddish brown. the only way to get rid of it is to cut it off or keeping dying the hair to keep the red out. Once you chemicaly alter your virgen hair it will never stay back to natural haircolor it will  fade and that red underlying undertone will always show up.

Not sure what kind of brown you are trying to achieve dark, medium or light brown?! Good news it it is always easier to go darker than lighter.Brown hair contains a lot red yellow and blue mix the three primary colors together = brown.

It is always good to understand your color wheel because it helps people understand how to color their hair and to get the shade they want to achieve

heres another great website it explains the color wheel pretty good

Hair Coloring Success - The Hair Color Wheel

The thing to remember is to do a strand test. Once your hair is chemicaly altered you hair becomes very pourous! Meaning it will suck up the hair dye like a sponge. also the recommended time is usually for people who are starting with virgen hair

Ash is suppose to counteract with orange. sometimes it works sometimes it doesn't. I had a haircolorist lift my hair it  orange wanted blonde but she didn't lift my hair to yellow not even close to pale yellow and she put a ash toner my hair turned green. Not green like like a leaf but green like a muddy greenish ashy brown color.

Anyways your hair will fade hopefully not it won't look so dark as the weeks go by.