Since, you have taken the time to reply and resurrect this old post... I'll continue to expand on my original reply.
It’s difficult to say what the root cause is, based on the limited description of the process, compound formulation, and the limited observations... any of which may be unrelated to the color shift. It would also be helpful if the original poster (OP) gave some feedback based on the suggestions given thus far.
Several things stand out in the OP's condition statements;
1.) He is comparing two compounds; non-lead and lead based stabilizer, and he give no indication that the formulations were altered to compensate for the switch from lead to non-lead.
2.) He is observing a color shift or color fade in the non-lead compound. And, he is seeing a temperature increase at the metering zone compared to the lead stabilized compound during processing.
I still believe, based on the limited information given that the non-lead compound is the cause of both the color and processing issues. First, it's the only thing that's changed in his process. And secondly, i have seen these same issues in many lead to non-lead conversions, especially where the tinting strength of the lead is not considered and where the superior stabilizing properties and unique lubricating properties of the lead compared to the non-lead are not addressed.
Lead has tinting properties similar to TiO2. And unless the non-lead formulation is adjusted for the lack of tinting or lower whiteness contribution of the non-lead, then the original color match is doomed.
The processing issues are either related to these lubricity or stability differences, and can only be solved by a re-balanced formulation. Fusion studies using a Brabender or Haake can go a long way toward understanding these fusion, stability, and lubrication dynamics.