filters (typically made by Kodak) used in evaluating a color print, in particular to determin which color is the dominant.
Remember that these filters, although they may be the right color, don't have the optical quality necessary in printing filters (the ones that form the filterpack).
In other words, if you use them for printing, the print will be slightly but disturbingly blurry and you will hate yourself.
The filters are normally mounted in cardboard holders. On one side of the holder you find the correction for negative printing, and on the other side you find the correction for color reversal printing, that's to say, printing slides to materials like Cibachrome.