I don't know what "dark" vs "maduro" means.
If you consider a specific variety of filler that is called volado, seco, viso or ligero (or even corona), in that sequence, the thinner, lighter-color leaf is from the bottom of the stalk, where the leaf contains the lowest levels of alkaloids. As we move up the stalk, the leaf becomes thicker, finishes to a darker color, and contains higher alkaloids. These terms are reflections of their actual stalk positions.
Wrapper "colors"are a visual rating of the finished leaf. An aesthetic choice by the individual doing the grading. While they sometimes don't correspond with their priming levels, the sequence is the same. Lighter, thinner, lowest alkaloid wrappers are from the bottom of that plant of that variety from that finca during that crop year, and the darkest, thickest, highest alkaloid wrappers (same qualifications) are from the top.
Sometimes the finished color from a particular variety (for example, Nostrano del Brenta) may range from dark EMS at the bottom of the stalk, yet produce nearly black, Oscuro starting at mid-stalk. So some varieties shift the color range downward on the stalk.
So the short answer to your question is that it's the plant, and more specifically, the leaf, not a difference in fermenting process.
Bob