Customizing the icon of a folder makes it easier to navigate through your filesystem, since you do not depend only on the name of the folder to identify it. In fact, it is usually easier to identify a file or folder by its icon than it is to identify it by its name. Dolphin makes it easy to customize the icon of any folder:
First, right-click the folder and go to .
Then go to tab, and click the icon in the top left corner.
A dialog window will pop up.
There, you can:
Once you find the icon you want to use for the folder, click it. You will be back to the Properties dialog window, and the icon you clicked will have replaced the old one.
Now, click to finally apply the changes for real. Your folder will be now using the icon of your choice.