The Collaborative International Dictionary of English v.0.48:

Colorless \Col"or*less\, a.
   1. Without color; not distinguished by any hue; transparent;
      as, colorless water; a colorless gas.

   Note: [Narrower terms: ashen, bloodless, livid, lurid, pale,
         pallid, pasty, wan, waxen; neutral; white] [Also
         See: achromatic, colorless.]
         [1913 Webster + WordNet 1.5]

   2. Free from any manifestation of partial or peculiar
      sentiment or feeling; not disclosing likes, dislikes,
      prejudice, etc.; as, colorless music; a colorless style;
      definitions should be colorless.
      [1913 Webster]

   3. having lost its normal color.

   Note: [Narrower terms: blanched, etiolate, etiolated,
         whitened; bleached, faded, washed-out, washy;
         dimmed, dulled, grayed; dirty; dull, sober,
         somber, subfusc] colored

   Syn: colorless, uncolored, uncoloured.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

colorless
    adj 1: lacking in variety and interest; "a colorless and
           unimaginative person"; "a colorless description of the
           parade" [syn: colorless, colourless] [ant:
           colorful, colourful]
    2: weak in color; not colorful [syn: colorless, colourless]
       [ant: colorful, colourful]