I agree with Tartan. If you can find a large scale print that incorporates all 4 colors (like your carpet does), that would be ideal for the sashing. Otherwise I'd probably go with a triad color scheme & pick a deep gold for the sashing (similar to the gold in your carpet). You could go with a solid gold or a small print.
The other option would be a slightly darker shade of one of the colors you have in there. I wouldn't pick the exact fabric because then wherever that fabric touches the sashing, it will meld together as one shape instead of maintaining the piecing design as is.