It's the same color as the GT's ground effects.
Color code would be on your SPID sticker in the front trunk.
Medium Gray Metallic - GM paint code WA7719
(was also a Corvette exterior color 84-87)
Dupli-color etc. should have a catalog to look up their match.
From “All 1984-1988 Pontiac Fiero Colors, Codes & Quantities,”http://www.fiero.nl/forum/Forum1/HTML/098118.html
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3.4L Supercharged 87 GT and Super Duty 4 Indy #163