I have heard that it is called dove gray but don't recall brands and numbers so I kept mixing enamel paints until I got as close as possible. http://www.geocities.com/jims68fb/Trunk.html
and... There are some people who have said theirs was black.
I'm pretty sure it is the same gray or silver that is on the spokes on the rally wheels which you can get in a Dupli-color. I do think they came either black or gray. Please correct me if I'm wrong
I can positively say the grayish color of mine is one of the correct colors. I've had the car since '69 and I know of another one owner '68 that is that same gray. I say "one" of the colors because I have heard similar discussions which surface more than gray. I believe there is more than one correct color.
Besides mine on the web site, here's one from a one owner POCI Concours perfect score '68:
Here is my best guess, if your car came with Rally's the correct color would be gray [the same color as the spokes on the rally. If the car came with steel rims with covers the space saver was black. Also, some of the jacks were ford blue and some were black