This method is the same thing as planar mapping along say, the X, Y or Z-axis. This map will color the entire head and act as the base material. In the next two map slots, inside the Composite Material, place two Mask materials. Each one of these will be used to map the sides and top of the head. Notice that the top slot is the image map and the bottom is the mask.
Lets look at the parameters for each of the images.
All of these parameters are the same as for the first image in the composite material with the exception of the mapping. I know it s not fancy, just wanted to prove a point. Now, in the mask section, I placed a simple black and white mask. Here are the mask s mapping parameters. Notice that they are identical to the map you are trying to mask.