I looked up your RPO codes, some I had to guess what it says so they might be wrong...
AU3 - LOCK CONTROL SIDE DR, ELEC
A01 - WINDOW TINTED ALL, SHADED WINDSHIELD
A28 - WINDOW RR FULL WIDTH, SLIDING
A31 - WINDOW POWER OPERATED, ALL DOORS
A52 - SEAT FRT BENCH
CD4 - WIPER SYSTEM WINDSHIELD, PULSE, VARIABLE DELAY
C60 - HVAC SYSTEM AIR CONDITIONER FRT, MAN CONTROLS
C7A - GVW RATING 10,000 LBS
DF2 - MIRROR O/S LH & RH, WIDE LOAD, FOLDING, STAINLESS STEEL
E63 - BODY EQUIPMENT FLEETSIDE PICK-UP BOX
F59 - STABILIZER SHAFT FRT
GMC - PLANT CODE PONTIAC, MI, USA
GQ1 - AXLE STD RATIO
G??
J55 - BRAKE SYSTEM HEAVY DUTY
KC4 - COOLING SYSTEM ENG OIL
K34 - CRUISE CONTROL AUTOMATIC, ELECTRONIC
K60 - GENERATOR 100 AMP
L19 - ENGINE, GAS, 8 CYL, 7.4L, 1TBI, VB
MX1 - MERCHANDISED TRANS AUTO PROVISIONS
M40 - TRANSMISSION AUTO 3 SPD, TORQUE CONV, VAR 1
NA5 - EMISSION SYSTEM FEDERAL, TIER 0
NJ8 - FUEL TANK 50 GAL (201 L) DUAL, RECT, STEEL
N31 - STEERING WHEEL HEATED
N33 - STEERING COLUMN TILT TYPE
N41 - STEERING POWER, VARIABLE RATIO
???
R05 - WHEEL DUAL RR
SLM - STOCK ORDER, PROCESSING CODE (???)
UM6 - RADIO, AM/FM STEREO, SEEK/SCAN, AUTO REV CASS (???)
UY7 - WIRING HARNESS TRUCK TRAILER, HD
U01 - LAMP FIVE, ROOF MARKER, TRUCK
U37 - LIGHTER CIGARETTE
U66 - SPEAKER SYSTEM 7, CUSTOM
U73 - ANTENNA FIXED, RADIO
VB3 - BUMPER RR STEP, CHROME, IMPACT STRIP
VG3 - BUMPER FRT IMPACT STRIP
VK3 - LICENSE PLATE FRONT FRT MOUNTING PKG
VR4 - TRAILER HITCH WEIGHT DISTRIBUTING PLATFORM
V02 - RADIATOR HEAVY DUTY, WITH TRANS OIL COOLER
V22 - GRILLE RADIATOR, CHROME
V73 - VEHICLE STATEMENT USA/CANADA
V??
XHP - TIRE FRONT LT225/75R16/D BL R/PE ST TL ALS
YD6 - SPRING RR, BASE EQUIPMENT
YD7 - AXLE (USED WITH R05) FOR SCHEDULING GVW PLATE
YE9 - CONVENIENCE PKG COMFORT & DECOR LEVEL #3
YHP - TIRE REAR LT225/75R16/D BL R/PE ST TL ALS
ZQ2 - SALES PACKAGE DRIVER CONVENIENCE
ZY3 - COLOR COMBINATION TWO TONE, SPECIAL
Z53 - CLUSTER INST, OIL, COOL TEMP, VOLTS
Z82 - TRAILER PROVISIONS SPECIAL EQUIPMENT, H.D.
18I - INTERIOR TRIM, DK CHARCOAL (I) (91) (???)
6XA - COMPONENT FRT LH COMPUTER SEL SUSP (6XA)
7XA - COMPONENT FRT RH COMPUTER SEL SUSP (7XA)
79A - STRIPE COLOR ACCENT, DK CLARET (97)
82D - TRIM COMBINATION CLOTH, MED DK GRAY (D) (91)
821 or 82I - INTERIOR TRIM (DARK MEDIUM GRAY) (???)
96U - PRIMARY COLOR EXTERIOR, ULTRA SILVER MET (02)
If it had a secondary color and it was black the RPO code would be 41L and it's not there... I don't know why the L9150 paint code is printed on your RPO sticker.
96U - PRIMARY COLOR EXTERIOR, ULTRA SILVER MET (02) has both WA8867 and WA8807 as paint codes so that should be right.
My guess is your truck was Ultra Silver Metallic with a dark claret (brown/purple/red-ish color, like red wine) stripe and medium dark gray cloth interior, like DieselDetroit said.
But I don't know why the ZY3 code is there (two tone, special) unless the stripe is counted as the second color... mine is solid blue and my RPO code says ZY1 (Color Combination, Solid).
I think you should go to a dealership and have them look it up... cos some things makes no sense.
Source: GM RPO Master List & RPOcodes.com and paintref.com for colors.