What can you do to remove red from brown hair in order to go a medium to dark ash blonde?

Strip your hair. I think you can buy stuff at the store but it's better if it's done by a pro cause it can damage your hair if done wrong. Or just wait till the red fades out. Once it fades out enough you could try dying it back to your normal color then blond.

What can you do to remove red from brown hair in order to go a medium to dark ash blonde?

Brown hair darker than medium or dark ash blonde, so using permanent hair colors won't lighten brown hair to blonde, but will turn it reddish brown. It must be bleached/stripped to take the brown to a light lemon yellow color and then can be dyed to blonde shades. Red tones will strip quicker than brown.

My hair was dark brown and hair dresser tried a test stripping but color wouldn't budge, told to dye it to a medium brown (roots), then have it stripped.

Clairol implied their strip products may be stronger than ones at hair dressers. I think so based on past experience. Depending on how light your brown color is; if dark brown or medium brown I'd use Clairol Ultra Light Blonde stripper, or with light brown could use Clairol Light Blonde stripper. If red is left along with the yellow after stripping, maybe more stripper on red only, but I would call Clairol's 800 # on the box. The longer the hair the more stripper needed, but if you strip it more than once or twice the hair could break off, stripping does damage hair. I tried many boxes of permanent blonde color once to achieve blonde hair but just got damaged reddish brown hair. If one wants to keep brown hair but get rid of red, then there are red removers at beauty supplies that have ash color base. Hope we get the ash blonde we both want.