The Preferences Panel allow to change some global setting in Plot
Time Format: Defines the format for time values used in Plot.
Default Font: Allow to define default fonts for axis labels and axis numbers.
Set Default Style: If clicked the style of the current working buffer in the current document will be the default style for newly generated data buffers.
Check for updates at startup: If checked Plot will inform you if a program update is available.
Here you can define your own default colors used by Plot. It is also possible to restore the default colors provided by Plot.