Highlights Add Dimension
Highlighting hair with bleach adds dimension. When dying hair, the overall result leaves a monotone color throughout the entire head of hair. However, when highlighting hair, the highlighted portions will be lighter than the base color. This creates dimension and subtle contrast. Bleaching hair with foils will create chunky ribbons of highlights while a highlighting cap will create more subtle color contrast areas.
Darker Hair Is Harder To Highlight
Highlighting hair involves virtually lifting away pigment using a mixture of bleach and hydrogen peroxide. The darker the hair is, the harder it is to lift as bleaching hair is done in levels. Dark hair must be lifted several levels from dark brown to light brown to red to orange to yellow. For instance, going from dark brown to light blonde takes lifting of several layers using bleach and a high volume peroxide in addition to another step to add color or tone the color. Processing hair in this manner is not only difficult to achieve, but it damages hair as well.
According to idyemyhair.com, regardless of the shade that you are attempting to acquire, do not leave bleach on your hair until it turns white. This does extreme damage to your hair beyond normal bleaching. Each hair type is different and there is no cookie cutter amount of time to leave bleach on hair. Test a few strands of hair first using powder hair bleach and a 20 to 30 level hydrogen peroxide developer in equal parts. In testing a few strands first, this will serve as a guide to the amount of time needed to achieve your preferred level of lightening.
References and Resourcesidyemyhair.com: Dying/Bleaching FAQ
Style-hair-magazine.com: Bleaching Hair
Hairboutique.com: Hair Color Tips