The colors you see in that list are defined in the Style settings. Click on a tile to change the color in the menu.
If you want to have a file that always starts with a different color pallet, try creating your own custom template and use that as a starting point for new files.
You can also click the gear menu in the top right of the Style settings to apply a color pallet across all graphs in your file at once, or copy the Style settings between graphs. To do that, select Edit > Copy and then click another thumbnail (in the same other other file) and select Edit > Paste.
Does that help?
P.S. You can also have data driven color schemes that you might want to explore. We did a webinar last November on that topic:
Webinar Recording: Color & Color Schemes