Only items you can activate from Finder > Preferences > Sidebar have custom sidebar icons. If you manually add folders, such as the Developer folder, to your sidebar favorites, they will show up with a generic sidebar icon, even if the folder has a custom Finder icon.

The only way to change this would be with hacks as described by e.g. [Keyboard_Gamer's answer][1].


  [1]: https://apple.stackexchange.com/a/283741/238578