I have a sales brochure for 1987 Spiders but it does not show the available colors. If you search here on the BB there are a few threads with color charts. I also found the colors for all Alfas for 1986 on Dr. Color Chip's website Select Paint Color - Dr. ColorChip: Automotive Paint Chip Repair Systems , but I am not sure how accurate it is or what colors were specifically for the Quad (as opposed to other Spiders or Milanos).
Chris Reese's Original Alfa Spider has paint descriptions:
Alfa Red AR530, Arctic White AR 230, Onyx Black AR 817, Red Amarillo AR 143, Cabernet Red Metallic AR 547, Argento Silver Metallic AR76, Stellar Blue Metallic AR370, Medium Blue Metallic AR940.
You can look at these colors at must places that mix auto paint facility.