When you start to frequently shampoo, discoloration will come. It also depends on what type of product your colorist uses. As long as you use a good, strong product, a professional product, the color will last longer.
Still, with that, if you don't have frequent color, like on the time schedule, discoloration can happen very easily. Bad shampoo can give a different tone of your color. If you go away on vacation and you're coming back, there's sun versus winter. There are so many elements in place that can create discoloration, so it's very important to keep up once a month, once every 45 days.