I've moved your post to it's own thread from an unrelated one.
The Serial Number won't tell you what the original color was. But if you look in a spot that hasn't been painted over (under the mirror bracket, part of the firewall, or between the cab and bed), you may find the original color.
HERE is a good resource for colors. But the original GM color code won't do you much good, as they're obsolete.
Once you determine the color, you can also go
HERE and find various paint manufacturer's color codes to match.
[on edit]
This might help as well.
