The Collaborative International Dictionary of English v.0.48:

colorful \colorful\ adj.
   1. having striking color. Opposite of colorless.

   Note: [Narrower terms: changeable, chatoyant, iridescent,
         shot; deep, rich; flaming; fluorescent, glowing;
         prismatic; psychedelic; red, ruddy, flushed,
         empurpled]

   Syn: colourful.
        [WordNet 1.5]

   2. striking in variety and interest. Opposite of colorless
      or dull. [Narrower terms: brave, fine, gay, glorious;
      flamboyant, resplendent, unrestrained; flashy, gaudy,
      jazzy, showy, snazzy, sporty; picturesque]
      [WordNet 1.5]

   3. having color or a certain color; not black, white or grey;
      as, colored crepe paper. Opposite of colorless and
      monochrome.

   Note: [Narrower terms: tinted; touched, tinged; amber,
         brownish-yellow, yellow-brown; amethyst; auburn,
         reddish-brown; aureate, gilded, gilt, gold, golden;
         azure, cerulean, sky-blue, bright blue; bicolor,
         bicolour, bicolored, bicoloured, bichrome; blue,
         bluish, light-blue, dark-blue; blushful,
         blush-colored, rosy; bottle-green; bronze, bronzy;
         brown, brownish, dark-brown; buff; canary,
         canary-yellow; caramel, caramel brown; carnation;
         chartreuse; chestnut; dun; earth-colored,
         earthlike; fuscous; green, greenish, light-green,
         dark-green; jade, jade-green; khaki; lavender,
         lilac; mauve; moss green, mosstone; motley,
         multicolor, culticolour, multicolored, multicoloured,
         painted, particolored, particoloured, piebald, pied,
         varicolored, varicoloured; mousy, mouse-colored;
         ocher, ochre; olive-brown; olive-drab; olive;
         orange, orangish; peacock-blue; pink, pinkish;
         purple, violet, purplish; red, blood-red, carmine,
         cerise, cherry, cherry-red, crimson, ruby, ruby-red,
         scarlet; red, reddish; rose, roseate; rose-red;
         rust, rusty, rust-colored; snuff, snuff-brown,
         snuff-color, snuff-colour, snuff-colored,
         snuff-coloured, mummy-brown, chukker-brown; sorrel,
         brownish-orange; stone, stone-gray; straw-color,
         straw-colored, straw-coloured; tan; tangerine;
         tawny; ultramarine; umber; vermilion,
         vermillion, cinibar, Chinese-red; yellow, yellowish;
         yellow-green; avocado; bay; beige; blae
         bluish-black or gray-blue); coral; creamy; cress
         green, cresson, watercress; hazel; honey,
         honey-colored; hued(postnominal); magenta;
         maroon; pea-green; russet; sage, sage-green;
         sea-green] [Also See: chromatic, colored, dark,
         light.]

   Syn: colored, coloured, in color(predicate).
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

colorful
    adj 1: having striking color; "colorful autumn leaves" [syn:
           colorful, colourful] [ant: colorless, colourless]
    2: striking in variety and interest; "a colorful period of
       history"; "a colorful character"; "colorful language" [syn:
       colorful, colourful] [ant: colorless, colourless]
    3: having color or a certain color; sometimes used in
       combination; "colored crepe paper"; "the film was in color";
       "amber-colored heads of grain" [syn: colored, coloured,
       colorful] [ant: uncolored, uncoloured]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "colorful":
   bicolor, bicolored, blatant, brave, brazen, brazenfaced, bright,
   bright-hued, brilliant, burning, colory, crazy, crude, daedal,
   deep-colored, dichromatic, divers-colored, exotic, extravagant,
   flagrant, flaring, flashy, flaunting, florid, garish, gaudy, gay,
   gay-colored, glaring, gorgeous, harlequin, high-colored, intense,
   kaleidoscopic, loud, lurid, many-colored, medley, meretricious,
   motley, multicolor, multicolored, multicolorous, obtrusive,
   parti-color, parti-colored, polychromatic, polychrome, polychromic,
   prismal, rich, rich-colored, screaming, sensational, shameless,
   shot, shot through, showy, spectacular, spectral, splashy, tawdry,
   thunder and lightning, trichromatic, trichromic, tricolor,
   tricolored, two-tone, varicolored, variegated, versicolor,
   versicolored, vivid, vulgar