Dictionary:

colorless

  (kŭl'ər-lĭs)

1. Lacking color.
2. Weak in color; pallid.
3. Lacking animation, variety, or distinction; dull. See synonyms at dull.

adjective

1. Lacking color: ashen, ashy, bloodless, cadaverous, livid, lurid, pale, pallid, pasty, sallow, wan, waxen. See colors/colorless.
2. Lacking liveliness, charm, or surprise: arid, aseptic, drab, dry, dull, earthbound, flat, flavorless, lackluster, lifeless, lusterless, matter-of-fact, pedestrian, prosaic, spiritless, sterile, stodgy, unimaginative, uninspired. See excite/bore/interest.
3. Without definite or distinctive characteristics: bland, indistinctive, neutral. See strong/weak.

Definition: unlively, uninteresting
Antonyms: brilliant, distinctive, intense, interesting, lively, stimulating, vibrant

adj

Definition: without hue
Antonyms: brilliant, colored, colorful, motley, rich, stimulating, vibrant, vivid

The adjective has 2 meanings:

Meaning #1: lacking in variety and interest
  Synonym: colourless
  Antonym: colorful (meaning #2)

Meaning #2: weak in color; not colorful
  Synonym: colourless
  Antonym: colorful (meaning #1)