Each area in the dreamscape has fog that blocks out distant geometry.
The color of the fog represents what area you are in for the map and the four corners of the chart. Dark colors usually mean you're in the less active areas in the map and they usually turn out to be in the downer/middle areas in the four corners, while the lighter, more sensual colors, represent the area as a highly active area and that it will most likely be in the static/upper areas of the four corners.
The amount of fog varies between dreams. It is unknown as to why this is.