The colors on a shaded relief map are based on the reflectance from the grid surface. Reflectance values range from 0 to 1. A reflectance value of 0 means that no light is reflected toward the viewer. A reflectance value of 1 means that all incident light is reflected toward the viewer.
The colors on the map do not represent slope, they represent reflectance. These maps use colors to indicate the local orientation of the surface relative to a user-defined light source direction. Surfer determines the orientation of each grid cell and calculates reflectance of a point light source on the grid surface. The light source can be thought of as the sun shining on a topographic surface. Portions of the surface that face away from the light source reflects less light toward the viewer, and thus appear darker. If you move the light source, you can see that the colors will completely change on the map.
If you like the shaded quality of the black to white (gray shaded) shaded relief map, but wish to apply colors and have the colors represent the Z values in the grid, use a color relief map instead. Surfer 13 and later versions support hill shading for color relief maps, and they are a very visually striking map, combining vibrant colors based on true Z values in the grid and hill shading giving a 3D appearance.
Updated November 2, 2017