Okay, that's a clue... Only in B/W mode it is possible to select 600dpi.
Despite what's in the user guide, the specifications read: (dutch prochure):
Zwart-wit en kleur scannen, 1200 x 1200 optisch, 4800 x 4800 verbeterd.
This implies that it should be capable of scanning colors 1200dpi optical.
I also found this in the online user guide page 252 found here.
Maximum Document Width -- 8.5 inches (216 mm)
Effective Scan Width -- 8.2 inches (208 mm)
Maximum Resolution --- • Optical: Up to 600 x 600 dpi mono, up to 300 x 300 dpi color and grayscale.
• Enhanced: Up to 4800 x 4800 dpi mono, up to 2400 x 2400 dpi color and grayscale.
I found this article for modifying the resolution when scanning for you to look at.
If you are still having issues after trying these steps, I suggest that you call into our support department for assistance @ 1-800-821-2797 or 1-800-835-6100
Using the flatbed scanner, I can scan up to 1200x1200dpi.
Using the document feeder scanning should be possible up to 600x600dpi.
However, in the CWIS settings for "Scan to Network" (and Scan to Mail etc.) the highest resolution possible is 300dpi.
How can I set this to (at least) 600dpi?