What makes black pepper strong or mild?
There are really only two main varieties of black pepper, Tellicherry and Malabar.  They are processed the same, and in general (though you might tell a difference in a side by side comparison) have the same flavor profile.  What could easily change the flavor and/or "spiciness" is freshness of the spice, and the grind (both how coarse/fine, and how recently the pepper was ground).  Further, heating the pepper in a pan before grinding, also brings out more flavor.  It also turns out that terroir is a factor in the flavor and aroma of pepper.  So, where your pepper comes from certainly impacts your experience.  The coloration is simply a composition of the inside and outside of the seed itself when it is ground and mixed.