Well,there is a solution to that. Just don't install the mod. If i'm understanding correctly, the blond hair imouto will only be applied if the FFFox mod + color mod is installed (the blond hair imouto is not available yet,for anyone reading this.)
Although I wonder if there could be an option to "turn on" the blond hair or not,once the coloring + mod is completed, for those who are not interested in changing the imouto's hair color.
Personally,i think the blond hair color is nice. The grey (it's more silver-ish) imouto is still cute too.