Subtle Shading? Strong shading? One of the awesome things about fountain pen ink is the endless subtle options.
What is shading? 
Shading occurs when letters or parts of letters have variance in ink distribution resulting in lights and darks ( more paper show thru) within a single line or letter.
Do nibs and paper play a role? Absolutely; wider nib- more line variation in color..  creating varying degrees of color itnsentity.
low saturation ink tends to have more shading where high saturation tends to have pools and sheening.