One thing i always find with classic cars that have so many options available for them is that you are always looking for a way to change your car believeing it will imporove the look of it.
My Highland green fastback has a black interior which i was considering changing to parchment to brighten things up. I then discovered that my interior was originally ivy gold, which i think is a very nice colour combination. When i discovered this i realised that everyone has a different opinion of what looks good but at the end of the day to have a classic mustang is so rare that all the interiors look good.
My suggestion, keep the interior you have. If its the way it came out of the factory that makes the colour scheme original. If its not the original colour then go for it, each to thier own.