Is there some kind of tweak I can do so that when I scan in full color, the white paper scans as white, not light blue or yellow?
Most likely you'll have to correct the color after scanning.
Scanners don't have the same color sensitivities as your eye, and won't see everything the same as you do.
And scanners don't adapt to colors like you do (they'll see the newspaper as gray on gray, while you see it as black and white).
Try using the white eyedropper in Levels on an area that should be white.
Along with Chris' explanation, it is also likely that the paper background has greater UV reflectance, that you scanner's chips see, but your eyes do not, and that can differ, paper stock to stock.
You can probably Save a Levels adjustment (say to Scanned_Paper), and reuse that to quickly correct future scans.
There are also some 3rd party scanning software programs, that allow one to adjust much more at the time of scan, than most scanning software, included with your scanner. However, those might well differ greatly (I'd suggest that you trial, before you buy), and some will be very expensive.
Thanks Chris and Bill. I have thousands of pages to scan so it's not feasible to use the eyedropper. Bill can you please explain more what you mean by a levels adjustment. What might be an example of that? I've been scanning using Twain at 256 colors which takes care of the problem, but of course the colors are not as accurate as in the original artwork. Thank you.
Ooops. What I'm scanning is color calligraphy and graphics and it seems to print just fine doing that, crisp and clean. I just wish I could find a solution to the uneveness of the color of the white background paper.
Doesn't your scanning software give you a white balance adjustment control?
I use VueScan on three different scanners. Setting the white balance is a one click operation, as is the focusing of scanners that have focusing capabilities.
Explore whatever scanning utility you are using now and if it doesn't have that capability look into VueScan or SilverFast.