Director Vincente Minnelli was known for his use of lush color, and Gigi is a great example. Color is everywhere. The green of the Bois de Boulogne, the deep reds of Mamita's house, Gigi's brightly-colored schoolgirl clothes, the colorful dresses on the ladies at Maxims—they all convey excitement and delight, even though they bore Gaston to death. That's his problem.
You'll notice that the less sophisticated women and ladies of the demimonde dress in the brightest colors, while the haut monde (fashionable, respectable women) are more subdued. The women in the park are in white and pale pinks, while Gigi's first glam dress is elegant white. Liane's dress at Maxim's is garish, definitely a comment on what Gaston thinks of her—common and crass. Gigi's simple but gorgeous white gown lets us know that she's different from the other mistresses.