What do the different colored overlays represent on the map?

The underlying map layer in GeoWarehouse is provided by Google Maps.  Google states the colours on their map are as follows:

• Green: Represents forests or other dense vegetation
• Brown: Indicates bare ground or rock
• Blue: Signifies water
• White: Denotes open terrain
• Black: Represents roads, trails, buildings, and other human-made structures

The subtle colour scheme (i.e., beige/tan) facilitates the easy differentiation between man-made and natural features.