As well as using shampooes and conditioners, there are also serval different masks you could use to preserve the the colors.
Try half and alvacado
1Tablespoon of honey
1 cup of regular non fat yougurt
1 teaspoon of cocunut milk or oil
(this mask is also great for your face!)
or take regular nonflavor coffee, when at room temp poor over your hair and leave in for 20-30minutes.