In trees only, please use a different icon (for folders only) and icon color to indicate an open node (see also attached screen shot):

• for folders only, use the filled folder icon (not the one showing the outline folder, as is the default).
• change the color of the icon to color #10 (see http://wiki.magnolia-cms.com/display/UX/Style+guides+for+visual+design)

Please note that there's a different issue requesting the default icon for folders to be the outline folder icon instead of the filled folder icon (see linked issues).