Oh, Boy....the choice of color....
Before I discuss "color", let me just add that when you read about the true rarity of these cars today, you'll want to go back to original--no matter what color your CS is, or was...
'68 Shelby owners would pretty much go back to original, regardless of the color, because of their values, and the importance of going back to what it was. Many classic cars have returned to their original color--regardless if it's popular today or not, because of that car's historical significance. We're approaching that "historical" thinking quickly.
I see some spirited ribbing here about the color red. Ferrari likes it, and "Little Red" was..umm.well....RED!
The color choices for 1968 Fords were chosen in mid-'67, and lime gold was popular (such as appliances, etc.), and one should consider the culture back then as to how the Ford color palatte was chosen.
There were a few of the Mustang colors that were also used on the Lincolns, and Cougars. EVERY color has it's place, and when you put it in context, you'll know that it's the right color. Sure, tons of old, chalky lime green Mustangs were painted red. Red seems like that sure-fire sportscar color, and maybe it's become a cliche'. That's not to say that Candyapple Red is less sophisticated than any other color. To say "sophisticated", it might mean colors like they use on BMWs, Jags or Aston Martins--which used on a '68 Mustang, would look out of place, or too custom by comparison to an original color.
However, ANY of the other colors, when they are in factory condition, are impressive. It's the Shelby fiberglass, and the stripes that will set off any color choice. The addition of styled steel Rally wheels, and perhaps a vinyl top in black or white adds even more. The interior color has a lot to do with how it looks, too.
It's all about context. 1968 context. Think of some of the 1955-57 T-Bird colors. "Coral" reminds me of thousand island salad dressing! But--it's a proper color for that period. Today, Ford brought back Lime Gold, with Legendary Lime. Same color. Go figure.
So, I'd go with the Brittany Blue. (psst...it was used on racing Ford GT-40s).
Paul N.
p.s. this discussion begs the additional question: "should I completely tear the car down and rebuild it?"...(after 40 years?)