Drop out ink is ink specifically colored to avoid reading in high-speed OCR

Optical character recognition

Optical character recognition, usually abbreviated to OCR, is the mechanical or electronic translation of scanned images of handwritten, typewritten or printed text into machine-encoded text. It is widely used to convert books and documents into electronic files, to computerize a record-keeping...

 scanners. It is often a pastel yellow, red or orange.

The purpose for dropping out specific colors is to allow the OCR scanner to ignore those colors and operate only on the foreground information.

Drop out ink is often used in the finance industry for automated paper invoice processing.

Drop out ink is NOT the same as inks that have been screened down.