Mixing drug store hair dye colors is OK to do as long as there is some careful thought put into the choice of dye colors. Two shades of hair dye can be mixed together to create a custom shade, but this technique is best left to those with confidence and experience.
Selecting home hair kit color shades requires one to put a lot of time and thought into which shades work best with a person's natural coloring. Hair shades on the box are not representative of the end result. The actual color may turn out slightly different than expected. The shade of the person's natural hair combined with the dye is what determines how the new color looks.