The Collaborative International Dictionary of English v.0.48:

Achromatize \A*chro"ma*tize\, v. t. [imp. & p. p.
   Achromatized; p. pr. & vb. n. Achromatizing.] [Gr. 'a
   priv. + ? color.]
   To deprive of color; to make achromatic. [Also spelled
   achromatise.]
   [1913 Webster]