Darker hair underneath makes the final look very natural.
Darker hair underneath on “Lightest blonde” and “Dirty light blonde” colors would mimic brassiness of the bleached blonde hair underneath as usually hair close to the neck is a little darker and more yellowish.
Darker hair underneath on “Blonde/brown mix” color would mimic the style when narrow highlights are only on the top area leaving hair underneath not highlighted. Usually it is the same brown color used in a mix.
Alternatively color of darker hair underneath can be matched to darker roots, low lights or other color chosen by a wig making artist so that the overall color blends nicely.