To customize the colours used by default in your organization, head to the Organization settings
—> Appearance
and you can update any and all of your default colours used in your organization.
You will see a list of colour palettes that you can choose from.
You can also create your own palette by clicking on the Add new palette
button. Here you can preview the palette before you save it.
Once you have created your palette, you can set it as the default palette by clicking on the Use this theme
button.
You can always come back to this page to change the default palette.
The next time you build a chart, it will default to using these colours for the series, unless you override them in the chart config.