Wide format scanning colors aren't always right

[litljay]

One of my clients has an old Contex 6010 wide format scanner that runs admirably well for its age and generally produces beautiful scans for them.

Most of what they scan is simple line work with a few colors; nothing elaborate. They do have one sheet that they want to copy (and there may be more like it in the future) that is a high-resolution overhead photograph of a freeway interchange with some engineering work done on top of it. At its highest resolution (600 dpi) the image is detailed enough, but it is terribly dark and much of the color is not right. Most specifically, some yellow areas come out green, no matter what we do. I can usually correct the darkness via brightness or gamma controls, but I can not get the yellow to appear yellow. Other scans with yellow in them come out fine.

I'm totally at a loss. Contex does not really support this scanner anymore due to its age, so updated drivers are not an option. The whole unit has been cleaned/serviced by a local pro so we can't blame a "piece of dirt".

Any ideas?

TIA

J