Xynk has a number of built-in color schemes which can be accessed via the "Scheme" popup in the format bar. The new color scheme will be applied to all graphs in the current file.
The default color scheme used in new files or with imported data can be set under Xynk -> Preferences -> Graphing.
You can also modify or create and save your own color schemes.
Just select a different color scheme from the "Scheme" popup.
In the Bar width field, choose or enter a value from 0 to 100%: