What is the average color of soil?
This gif, prepared by the United States Department of Agriculture - Natural Resources Conservation Services (USDA-NRCS) soil scientists at the National Soil Survey Center, has soil colors based on the Munsell Color System for the United States at different depths:



The soil colors nearest the surface are darker due to more organic matter and are lighter at depth with varying colors by region.

Source: http://munsell.com/color-blog/soil-colors-national-parks-anniversary/

This link also has soil colors of select United States National Parks.  For example: