The Collaborative International Dictionary of English v.0.48:

Achromaticity \Ach`ro*ma*tic"i*ty\, n.
   Achromatism.
   [1913 Webster]

WordNet (r) 3.0 (2006):

achromaticity
    n 1: the visual property of being without chromatic color [syn:
         colorlessness, colourlessness, achromatism,
         achromaticity] [ant: color, coloring, colour,
         colouring]