The Farnsworth D-15 test (D-15) is commonly used to screen for moderate to severe congenital color vision deficiency. The aim of this study was to establish reliable D-15 statistics for normal, deutan and protan subjects, and to investigate the different visual signals one can use to carry out the test, even in dichromats and rod monochromats. Six hundred and seventy-four subjects were examined using the D-15, the Colour Assessment and Diagnosis test and the Nagel anomaloscope. A rod monochromat and five dichromats were tested using the standard D-15 protocol before the caps were separated into two groups and subjects were asked to repeat the task. D-15 spectral radiance data, measured under D65 illumination, were used to estimate differences in photoreceptor excitations for each of the caps. When no crossings and up to two adjacent transpositions on the D-15 results diagram are accepted as a pass, 100% of normal trichromats, 54% of deutans and 43% of protans pass the D-15. A rod monochromat and two protanopes and deuteranopes were able to complete the D-15 when the caps were separated into two groups, despite severe loss or even complete absence of color vision. When up to two adjacent transpositions are accepted 50% of color deficient subjects, some with severe red/green loss, pass the D-15. While the D-15 is normally used to screen for moderate to severe color deficiency, subjects with severe loss can still use combined, residual red/green, yellow/blue and luminance signals to pass.