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The important knowledge about scans

Color should be adjusted 2 times

1) while (or after) scanning
2) after (or middle of) filtering

In the 1) process, don't clip out colors, and keep saturation low.

sample color histogram:


the image is already adjusted white/black balance, and also gammas. but levels aren't adjusted tightly because of the following reasons.

1) some filters couldn't work well at 0 and 255
2) filtering changes colors a bit, thus color adjustment can't be done before filtering.
3) but if colors are clipped, no more can be adjusted.

Color adjustment is one of the most hard part of scanning. but in any case, don't clip out colors before get the final best.
if colors aren't clipped out, then someone (or you in the future) can fix them sometime later.