I would think sanding residue would fill the pores and slow the coloring. I simply do not understand why anyone would allow any cake in a meer they want to color and enjoy the "benefits" of the mineral.
Now used, unkempt pipes? That's an entirely different problem. I think I'd still take a sharp edge and simple pare the cake slowly until I was back to the rough, natural surface of the mineral. Meerschaum isn't smooth to begin with. The outer surface of a pipe has been treated, therefore it is smooth. The inside is rough to the touch. At least all of my meers started that way. I haven't purchased a new meerschaum in many years.
I have a couple darkened enough, full of the residues that the coloring process has slowed to a "snail's pace." But, they have all been smoked well over 400 bowls. So, from my observations, admittedly anecdotal, the coloring hasn't ceased.
The changes are nearly imperceptible.