Closing the CMYK palette, opening the RGB palette and then saving settings as default will not change the palette of the Color Dropdowns e.g. the one on the Smart Fill Property bar.
To set the RGB palette, for example, as default for all Color Dropdowns, open the Default RGB palette and choose Set As Default from its Context Menu.
To open the Default RGB palette, click Window > Color Palettes > Default RGB palette.
To reach the color palette Context menu, do one of the following:
- Right or Left-click the "Palette Options" swatch (the first swatch:
in X3 and 
in X4)
- Right-click anywhere on the border of the palette
- Hold the right-button down, on any swatch, for at least 1 second (see note 2 below)
Notes
- You can easily create, edit and manage color palettes through the Color Palette Browser (Window > Docker > Color Palette Browser).
- Check the Secondary Mouse Button options under Tools > Options > Workspace > Customization > Color Palette.