Wratten filters are used to produce these colors and mixtures because they are the purest and most adequately documented color slides. The primary colors are three of these filters that are positioned near the corners of the chromaticity diagram. The colors to be matched lie along the lines between the primary filters, so that they can be readily produced by mixing various levels of the primary colors.
Included with this demonstration is a transparency of the chromaticity diagram, seen at the right above, showing where all of the filters used in this demonstration lie.