Use the Labels feature to add colored labels to your pictures, to help sort them by a system of your own.
To add labels to pictures, use the Descriptions Panel, or use the Information | Image Information [Shift+Enter] window's Descriptions tab. Alternatively, use the Browser's Information | Label submenu or the [Shift+1] through [Shift+9] hotkeys. To remove any existing rating from the picture, click None or use the keyboard shortcut [Shift+0].
The label (if any) for a picture is indicated in its Browser thumbnail, by an icon at the bottom right. Right-clicking that icon calls up a menu where you can change the rating or remove it (the "-" item).
To set which colors are used for labels, use Settings | Preferences | Labels. Here you can define up to 9 custom labels or choose colors used in other programs. Label information is stored within a picture's XMP-type picture information.