Dropout colors refer to colors that do not appear (or have difficulty appearing) in an image when a document is scanned using monochrome or grayscale settings.
For example, when scanning a document with black characters and a red frame, if red is selected as the dropout color, only the black characters will be scanned. (Sample images below)
The dropout color can be selected from [Custom] (multi-color), [Red], [Green], [Blue], [White], [Saturation], or [None].
Note: The effectiveness of the dropout color depends on the optical characteristics of the scanner, and the wavelengths of light with high sensitivity and the light sensitivity range vary depending on the scanner model. Therefore, even if the same document is scanned with the same dropout color, the dropout results may vary depending on the scanner model.
Reference: Examples of optical (spectral) characteristics of scanners
If dropout does not work, adjust image parameters such as Threshold to be brighter.
Reference: Example of changing image parameters (PaperStream IP)