My hair is dark brown and I dyed it a few times medium golden brown to lighten it a bit. Three weeks ago I dyed it an auburn brown just to put some red tones in it. It was okay but has faded way to fast. I would like to try another color but am afraid of it going darker. Can I use colorfix on my hair and then try a lighter red color? Or should I just dye it a medium reddish brown and hope for the best.

Don't change hair color at all. Most people look best with natural haircolor. (even actresses on tv)

To go from dark to a decent red shade, it's best to pre-lighten the hair to reddish level and deposit the desired color. But you can use a single process hair color with a higher strength peroxide to achieve a red shade. I suggest using a 20-volume peroxide developer with your selected hair in equal parts to lift the color a couple of shades. Be sure to use a shampoo and conditioner designed for color-treated hair to help keep the color from fading.