Yup, light grey, but like Stroked and Blown (S&B) said, the only thing different about a 7-up is being able to say it's a 7-up. I need to replace my carpet, but I'm going with black and it won't change the resale value at all other than maybe raising it for new carpet. But none of that matters to me since mines not now or ever will be for sale. My 7-up is my first stang and I've had a couple more since then (Currently own 3 have had 4 total) but I will always keep my first.
Pick a color you'll be happy with, it won't change the value.