MikeV
X-31 is acolor called "Titanium Gold". I checked ipmsusa.org for color reference links and none of the links has a match. To my eye a mix of gold and silver will get you close, probably about 50/50. If you go to tamiyausa.com you can download a printout of the tamiya color range. It won't be perfect but it should help.
(Originally, I said jet exhaust and silver. I came back to correct it but I noticed now that somebody beat me to the correction)