The paint code for your MX5 can be found be checking the VIN plate at the rear of the engine bay near the drivers' side or checking the inside of the drivers' door shut for a factory applied sticker.

12K    Twighlight Blue
HU    Neo Green (aka British Racing Green)
6W    Laguna Blue (aka Perky Blue)
SU    Classic Red
A1Q    Merlot Pearl Clearcoat
5N    Brave Blue Pearl Clearcoat
3L    Silver Stone
DU    Mariner Blue (aka French Racing Blue)
A5    Satellite Blue
11Q    Excellent Pearl Green Clearcoat
A3E   Eternal Red
27C Nordic Green Mica
25E Strato Blue Mica
28W Radiant Ebony Mica
HZ    Sunburst Yellow
PZ    Brilliant Black (Mk1)
A3F   Brilliant Black
11R    Sparkle Green Metallic Clearcoat
18G   Racing Silver
PT    Chaste White
18J    Grace Green
M8    Montego Blue Clearcoat
32V    Copper Red Mica
22A    Supreme Blue
24A    Starry Blue Mica
32S    Galaxy Grey Mica
25R    Splash Green Mica
25F    Garnet Red Mica
A3D    Pure White
22V    Sunlight Silver Metallic
23C    Crystal Blue Metallic
18K    Evolution Orange
27A    Velocity Red
A5M    Marble White
29Y    Titanium Grey
27B    Winning Bue Metallic
A3F    Brilliant Black (Mark 3)