The Collaborative International Dictionary of English v.0.48:

Achromic \A*chro"mic\, a. [Gr. ? colorless; ? priv. + ? color.]
   Free from color; colorless; as, in (Physiol. Chem.), the
   achromic point of a starch solution acted upon by an
   amylolytic enzyme is the point at which it fails to give any
   color with iodine.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

achromic
    adj 1: having no color [syn: achromic, achromous]