Since the color in scanned images is rarely even, a threshold value is often needed when defining colors for individual fields. The threshold value determines the size of the range of colors that the system will accept in relation to the reference color.
For example on a scale of 0-255 (the scale varies depending on where you are specifying it), a value of zero will only accept exact matches of the reference color, whereas a value of 255 will include a much broader range of colors.
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