Hair Color Red to Brown

I have had my hair medium to light red for about a year now and I would like to make it closer to brown again. I want to keep it at the same level as it is now, but I would like to take most of the red out but not all. I want it to be medium to light brown with very slight warm undertones. What color should I use? (e.g. ash, warm) I have a formal event to go to in about two weeks and would like to have it done by then so it looks nice. I haven`t colored my hair in about 6 months so I think it would be alright on my hair.