There is a review of the Sigma DP3 Merrill out there which makes an unsubstantiated claim (no evidence whatsoever) that the color in the Sigma DP3 Merrill is improved over the DP1/DP2 Merrill: “The DP3 Merrill's colors are more accurate (by far); the color modes do not work like older models leading to even more room to work your raw files”).
I have not yet observed a color difference among the DP Merrill cameras, and I have all three of them. If I were to make such a claim, I would back it up with evidence as a matter of professionalism.
I asked Sigma about this.
...not aware of any specific changes in the DP3M that would result in this effect.
Now we all know that companies can do things and not self-inform, and so I have asked for an official statement on the matter, just to be certain.