Putting food coloring in your hair isn't the best way to color it–it is very drying and isn't meant to be used on hair. If you regret putting that blue food coloring on your blond hair or it has been a while since you did it but you still have noticeable color residue in your hair, there is a way to remove it–lemon juice. Be warned: This will take a little time and a few tries before the color comes out completely.
Mix 2 parts lemon juice with 1 part water in a spray bottle.
Spray the blue parts of your hair with the lemon juice water and make sure it is saturated. Leave it on about 30 minutes.
Lean your hair into the sink or bathtub and rinse the lemon juice water out. Don't let it get in your eyes and try to keep it off your skin as much as possible because it is very drying.
Spray your wet hair with leave-in conditioner and brush it to get the tangles out after it has dried a little. Lemon juice will dry your hair out but the leave-in conditioner will help restore moisture.
Repeat these steps every other day for as long as it takes to get the food coloring residue out of your hair. Let your hair rest a day or two between tries.