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]

Moby Thesaurus II by Grady Ward, 1.0:

128 Moby Thesaurus words for "colorless":
   abstract, achromatic, achromic, aloof, anemic, arid, ashen, ashy,
   barren, blah, blanched, bland, blank, bled white, bloodless,
   blurry, boring, cadaverous, characterless, chloranemic, cold, dead,
   deadly pale, deathly pale, detached, dim, dimmed, dingy,
   discolored, disinterested, dismal, dispassionate, doughy, drab,
   draggy, drearisome, dreary, dry, dryasdust, dull, dusty, effete,
   elephantine, empty, etiolated, exsanguinated, exsanguine,
   exsanguineous, fade, faded, faint, fallow, feeble, flat, ghastly,
   gray, haggard, hazy, heavy, ho-hum, hollow, hueless, hypochromic,
   impersonal, inane, inexcitable, insipid, jejune, lackluster,
   leaden, lifeless, livid, low-spirited, lurid, lusterless, mat,
   mealy, milk-and-water, muddy, namby-pamby, neutral, obscure, pale,
   pale as death, pale-faced, pallid, pasty, pedestrian, plodding,
   pointless, poker-faced, poky, ponderous, prosaic, prosy, remote,
   sallow, sickly, slow, solemn, spiritless, sterile, stiff, stodgy,
   stuffy, superficial, tallow-faced, tasteless, tedious, toneless,
   uncolored, unimaginative, uninspired, uninteresting, unlively,
   vacuous, vague, vapid, wan, washed out, washed-out, waxen, weak,
   whey-faced, white, wishy-washy, withdrawn, wooden