I use M&P in alot of my designs. No problems with the m&p colors staying true if you used non-fade coloring in the m&p....if not you'll get sort of a "halo" around them later but you can use that to your advantage with a.......... just went completely brain dead...need more coffee....what the heck is it called, that hair coloring process where one color fades into another???
You may get some sweating on the exposed m&p during the cure period....just wipe off...it will stop. Make sure your m&p pieces are firmly embedded into the CP or they may separate when the CP shrinks. Spray liberaly with alcohol before embedding.
The best use of m&p in CP is being able to soap light scents. I use m&p as the colored swirl in my CP loaf....that's where I add the scent...& it sticks for months! Pic is "Rose Jam" scent...very soft & powdery FO....red swirls are m&p to which I added the FO. You have to work quickly so no elaborate designs, but it opens up new doors if you're willing to experiment. Use good quality m&p...you'll have very little to no sweat problems....blends better with CP...I use the natural by Catalina.