Color Specular Map helps the overall richness of the surface. Specularity is the light reflecting off an object. It's essentially a fake reflection. With a black and white spec map you always get a white highlight. Coloring your spec map gives this faked reflection different colors besides white. In general, I like to color the spec map for metallic surface (compliment, or split compliment with some other color details on the piece) to give it that extra diemension.
To test the result on how colorS will play off on the Diffuse, use "Linear Dodge" in PS blendmode for additive blending. You can use it to see what color your specmap will produce when added to your diffuse map. Turning off the diffuse map and view only D and N will give you an idea of how the specular reflection looks.
To calculate an additive blend yourself, just add the components of two colors. Red(255,0,0) plus Green(0,255,0) equals Yellow(255,255,0). You can see this in life by shining colored lights on a white surface, and causing them to overlap each other.