Seem a bit complicate to change the color, think you have to rebuild nautilus for that, but maybe there is another solution. Assuming you're using the default Yaru theme you can change the icon file with a different shape.
Ideally make a copy of the icon set because your changes might be overwritten when the theme's files are updated.
sudo cp -r /usr/share/icons/Yaru/ /usr/share/icons/Mytheme
Change the NAME (second line) with your theme name (e.g Mytheme)
sudo gedit /usr/share/icons/Mytheme/index.theme
And use gnome-tweak-tools to choose your own icon set.
sudo apt install gnome-tweak-tool
Launch "tweaks" and under "appearance" / "icons" choose your theme
Download shape you want, here it's a full circle :
wget -P $HOME/Downloads https://upload.wikimedia.org/wikipedia/commons/a/a0/Circle_-_black_simple.svg
Backup old icon
sudo cp /usr/share/icons/Mytheme/scalable/status/non-starred-symbolic.svg /usr/share/icons/Mytheme/scalable/status/non-starred-symbolic.old
Change with the new one
sudo cp $HOME/Downloads/Circle_-_black_simple.svg /usr/share/icons/Mytheme/scalable/status/non-starred-symbolic.svg
If you want to change the starred status it's "starred-symbolic.svg". Log out to take effect.