I learned here that, in addition to the effect of coloured pencil being a translucent tool, laying one colour on top of another pushes the underlying pigment into the white spaces inherently left by the original pencil strokes.

Thus when I used a light grey to make the lighter brown parts of his fur duller, it made it appear smoother and only more saturated with brown rather than duller with grey. I did use that to my advantage by going over the dark fur with the same light grey for a smoother muzzle.