This summer's biggest hair trend I've seen is called OMBRE or MELTING. Its popping up in hair collections everywhere.
"Ombre" literally means gradation in French, which refers to the dark-to-light fade of color that is used in the technique. The term was taken from the fashion world, where the ombre style of diffusing color is used for fabric, and is most often seen in kimonos. Instead of color that starts at the root, darker roots blend into a lightened mid-shaft and end. (www.stylelist.com)
Ombre can be either soft and subtle like Rachel Bilson's natural-looking locks, or play more progressive and dramatic. Either way, the shot of lightness adds a youthful look to hair, as strands naturally lighten under the sun further down the shaft from the root.
The initial process can take longer than a normal color because the bleach/color must be blended perfectly--but the resulting maintenance/touch-ups are much less often compared to to-the-root hilights.