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It is possible to define a custom color and linestyle for every threshold. The file TimeSeriesDisplayConfig contains a specific element, thresholdDisplayConfig, in which for every threshold a linestyle and color can be defined. More information on how to configure the thresholdDisplayConfig can be found at 02 Time Series Display Configuration, under ThresholdDisplayConfig.
Note that if no color is configured for a threshold, the time series dialog option to show the threshold colors in the background will be silently disabled (the user can still toggle the option, but doing so will have no effect, the background will remain white). Therefore, it is advisable to always configure colors for the thresholds.
Colors to be used can be found here.