Go back to the salon and ask them to fix it for you, if it's very bad.
Do you use a salon quality purple shampoo for blonde hair? I like Osmo and Fudge shampoos for decent quality without breaking the bank. Your salon should be able to offer a good recommendation.
If you use a harsh detergent shampoo such as the cheapy ones found in supermarkets or Boots etc., they will remove the tone from any highlights very quickly.