The two-tone colors are Shale (light) and Neutral (dark). The only C5s to use those interior colors were the 2003 50th Anniversary Edition, and the 2004 Collector Edition.
As I understand it, the shale color was the same between both, but the darker neutral color was a bit different. You probably got a piece from an 03 AE, and the only way you'll find one that matches must come from an 04 CE.
What you may want to do, is contact
Dennis at Double D. His business specializes in custom interior consoles, no doubt he would be aware of the color variance and likely has experience color-matching it. The sill panel you received could probably be refinished in the correct color, rather than trying to find the rare part.
