To edit the colour palette, go into edit mode, click the Panels button on the toolbar, and click the Colours button, whose icon is shown in figure 1.
Figure 1. the Colours icon
The Colours panel will then open in a new browser tab, displaying the current colour palette at the top and a colour picker below it.
To add a new colour to the palette, use the colour picker, as shown in figure 2, to choose the colour and then click Add this colour.
Note that colour palette is loaded at the time you enter edit mode. After adding a colour you will need to exit edit mode and then enter again in order to be able to use the new colour.
To delete a colour, click on its swatch and then click Delete this colour. After deleting a colour, all content on the site using the colour will revert to its default colour.