It is done with a ferric chloride solution, the softer metal dissolves faster leaving the harder metal sitting higher. Good basic explanation here... Shining Wave - Technical Information about Mokume-gane
After briefly reading that, I don't think that you can use that technic with M3. M3 is not real Damascus steel or Mokume Gane. It is similar in looks but is actually more like a powdered metal stabilized into blocks with some form of acrylic/plastic resin. The densities would be different between the two colors but the resin that bonds the metals would probably devolve rapidly. It might be worth a shot, but with the material being so expensive, and a very good chance of bad results... I think I'll pass this time and do a little more research.
Just thinking about it and a sand blaster would probably create the same effect without destroying the resin. A sand blaster is on my list of tools to buy when we get more space.