The 6 digit DSO and blank color indicates a special color Mustang. The '66 & '67 HCS cars had 6 digit DSO's. Same with the '68 Gold Nugget Specials, '67 Lone Star Limiteds, '67 Ski Country Specials, Color of the Month cars, etc.
I don't think there's a master list anywhere telling you what DSO equals what special color. They were just order numbers and got reset each year. Some cars that were produced in batches (like Gold Nuggets for example) would share a 6 digit DSO, but if you just walked in to the dealer and said you wanted a special order Mustang painted school bus yellow, your car would have been given a unique DSO.