In Qlik Sense, there are situations where one has a long list of items (say Brands, or SKUs) initially displayed in a line chart and when you filter them down, the colors don't get re-assigned which leads to the filtered chart showing several lines in the same color. This is a known limitation, however, it has quite a big impact on certain Dashboard designs.
There is color by rank, however, that work around has problems with displaying a legend and is quite rigid when you consider all the possible combination of filters possible.
The idea is for QS to be able to re-assign colors in a line chart on the fly as the data filter selection change.