I got my highlights done for the first time in about a year this month.  My hair seemed quite yellow and brassy afterwards.  This purple shampoo saved it!  The dark purple colour cancels out the yellow/orangey tones.  It's perfect for blonde hair but can be used on any hair colour if you're looking for a cooler tone.  I would recommend buying the 100g or 250g bottle of this as it should only be used about once a week.