Ash is burned leaf. A long, light-colored ash can mean the cigar is burning evenly (burn) and was bunched well, but it can also simply mean you have not tapped it. A flaky, dark ash can mean the cigar is burning unevenly or is too moist. None of these signs is a quality score.
The wrapper, binder, and filler all turn to ash together, so you cannot tell where the tobacco came from, or which part of the plant it came from, by the color of the ash. Let the ash fall naturally. Keeping a three-inch ash on the cigar is a trick, not a sign of a good blend.
If the burn tunnels into the center, or the ash forms a shell around a hollow core, check the draw and how the cigar was bunched; the color of the ash will not explain it.