Kodak i800 Series User Manual page 63

Background this value will be substituted in the grayscale
(pre-thresholded) image for the color being removed. Therefore, this
value should be higher than the threshold value selected on the Imaging
tab for this pixel to become the background color. For example, if you
are scanning a white document with a green form and you have selected
a bi-tonal threshold value of 127, you should pick an electronic color
drop-out background value greater than 127 so the substituted pixel will
be white in the dropped-out image.
A-61510 January 2005 3-23