Workbench Lists/Trees should display an Icon inserted right before the item name in the first column.

This icon should be configured per item-type (see WorkbenchDefinition.mainItemType / groupingItemType), through "icon" property.
JCR Properties should also be displayed with an icon (those are shown if WorkbenchDefinition.includeProperties is set).

Ideally, these icons should be displayed with the icon-font technique; configured icon value should then be just a CSS class name such as "icon-node-content", "icon-node-data" or "icon-node-folder".

Those icons may be created through the Magnolia Icon/GwtIcon widget classes.

Finally, here is the visual style guideline for positioning:
http://wiki.magnolia-cms.com/download/attachments/53253765/Desktop01_20_Pages_1.png
Icon color shouldn't be set, it should inherit grey color from cell text styles.