I made small project in order to reproduce the issue, but I couldn't. When a new Line Series is added, the previous one preserves its color, and the new one takes the next color from the palette. Note that the RadChart exposes the PaletteBrushesRepeat
property, which by default is true. This means that when the number of series exceeds the number of user defined palette brushes, the coloring will start from the beginning of the palette brushes collection. If the property is set to false, the next colors will be taken from the theme palette.
I have attached the sample project for reference.
the Telerik team