paint color w/o build sheet or fender tag?

Most re-paints are less than 100%. If I wanted to know the original color, I'd look inside the doors, underneath the carpet, behind the rear door panels, etc. Unless it was stripped to bare metal with the prior paint jobs, the original color will likely still be underneath parts of the interior. Underneath weatherstripping is a good place to look too. Even a shop that will spray out the jambs,engine compartment and trunk will often times leave everything covered by the interior.

If that doesn't work, you can also try slowly sanding down a portion of the exterior. With something like 180 or 220 grit, you will be able to see the color changes as you go down through the layers. The last layer before the primer should be the original color. Again, while that will work on a lot of paint jobs, if it was ever stripped to bare metal you're out of luck.