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
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I know about that function. It's mostly just about if there were any other options. I mean, I can find a folder icon pack probably, but that would make the ones I wanted customized but and also not match. For example, if I did a green folder and it was not flat like the Breeze ones are then it would look out of place until I replaced everyone with the one from the pack.
I think Breeze already includes folder icons of all colours, you just have to find them with that icon picker.
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