In the nearly 30 years of coloring my hair I have realized one thing - red is the worst color to get out if you don't want it that color anymore.
The lighter brown I used was nearly the same color as my natural color but as it has faded out the red has begun peeking through again. This time I'm going for a darker brown. It's going to look severe at first (though no more so than when I was coloring to black) but I think it will gradually fade to the color I want.
So . . . here I sit with a head full of chemicals for the next 30 minutes. Whoopie. Here's hoping the red will be covered over so the brown doesn't turn brassy as it fades.