While going through the FD RX-7's data, I made a discovery: One-off colors ala BMW Individual, 6 years for BMW did it. Here is the message I kept finding in both the FC and FD datafiles:
Special Color for Passenger Vehicle with Exclusive Color. (特殊色でお人車両の専用色〉)
This car is a special body color vehicle requested by the customer. (この車はお客様要望の専用ボディ色車両です。)
Please note that body color parts may require painting in a special color. (ボテディ色部品は専用色の塗装が必要な場合がありますのでご注意ください。)