The key problem with Prell and dish soap is that while it might "eat" any color off the cuticle...it will also "eat" natural oils which then may cause the cuticle to dry out and cause breakage or other complications.

The problem with semi-permanent color...when you go from light to darker is that even though it is supposed to "fade" over time, if your natural hair is exceptionally light OR it is porous from other chemicals or damage, the semi-perm color will "stain" your hair and ultimately what was meant to be semi-permanent will be permanent. 

Since you don't want to dye it again or put chemicals back you have limited options which ultimately boil down to:

1.  Getting as much of the dyed part cut off as possible and start over.  If you get your hair cut and want to minimize the rootage, ask for a style that is choppy or layered so that you can style your hair to minimize the obvious root growth.
2.  Strap yourself in and wait out the regrowth period and avoid adding any more color.  Have the ends trimmed to help equalize the colors.
3.  Invest in the latest head wraps, caps...etc., to camoflauge.
4.  Use headbands (they are really hot right now - especially bandeus) to disguise roots.

IF you decide to opt for chemicals....

1.  Ask a professional colorist if a color remover will help to pull out your current darker shades.
2.  Do with bolder streaks and highlights in a color close to your original base color.  Lighter blonde highlights around your face and hairline will blend with your natural hair as it grows out.

Growing out haircolor can be hell.  Hang in there.