The same color may be used twice in a graph when the Classic palette is selected and this may be a little confusing.
In the ‘Classic’ palette, the color order is always the same: ‘light blue’, ‘dark blue’, ‘light purple’, ‘purple’, ‘light yellow’, ‘yellow’. Then again ‘light blue’, 'dark blue' etc.
Each time-series displayed receives the same color over time. As long as the number of time-series doesn't change the colors stay the same and you will be able to follow the same lines when you open your graph again later.
We also have other palettes that will not use the same color twice. You may try ‘Orange’ or ‘Grey’ for instance. But 'Classic' is the only non-gradient color palette.