I have used one. I started with a new IMP and was faithful to the coloring bowl for about one year. In that time It colored very evenly but very slowly. The whole thing I found to be a pain and gave up, never to use it again.
I've never tried one but they seem like a lot of effort to get even colouring on a pipe. If that's what you're after then I say go for it!
In my experience, I think the main thing preventing good colouring of meers is that we have too many of them. I have a lovely Peterson meer estate that was clearly the only pipe of someone and has nice dark even colour all over. All my pipes haven't coloured much more than the shank but that's because I have far too many and rotate constantly!
I didn't even know coloring bowls were a thing, and I have a handful of Meers. What principle is it alleged to work on? I always figured the unique coloring patterns on my meers is what makes them, well, unique.
I recently saw a Meer that has had 60 to 70 bowls smoked though a coloring bowl. The color on the complete bowl was uniform.
Consider this while smoking a coloring bowl the Meers chamber becomes a large reverse Calabash . The smoke and moisture come into the chamber without the heat in which will allow the tars to absorb into the material. As we all know there is some heat in the smoke, this warmth helps to open the pours so the tars, nicotine can absorb. Yes, this is a lot of work to do but if you were to do this it would be just as unique as your other Meers. Another down side is that the Need should be new or very lightly smoked. In other words very little or no carbon in its bowl for the absorption.
I too have many Meets that don't get smoked they get forgotten because they are set aside so they don't get broken. Or in our busy lives they are set aside because we don't have the time to sit down and pay attention while smoking them. Last count I have about 12 Meers.
I remember seeing pictures of meers with a good dark patina cross-sectioned. The inside of the pipe itself was pristine white - only the outside surface of the meer holds the patina. This has led me to wonder if the Meer's patina really comes from the smoke being absorbed by the smoke inside the pipe, or if instead the patina is developed from just the smoke blowing around the pipe. If the color is being absorbed from inside the pipe, how can it be that only the outside of the pipe retains the patina?
I have a Peterson Sherlock Holmes meer, and always clench the pipe on the far left side of my jaw. The right side of this pipe, where the smoke would blow from my my mouth, is noticeably darker than the other.