Not sure if this is what you’re asking for but as with Dolphin 25.04.0 I can right-click a folder, select “Properties” (not sure if that’s the actual name in english; or just hit Alt+Enter), click/tap the folder icon within the modal and search for a color in the search box - lots of tinted folders (but not so much colors, though) shipped with the default Breeze icon theme
I don’t think there are any other options - as I understand that would require coloring an icon on the fly, wether a default icon or a custom icon