In Harry Potter and The Order of Phoenix, Dolores Umbridge always seems to be wearing pink color dresses. Is there any significance for this?
Dolores Umbridge is depicted as an extremely girlish character.
From the Harry Potter Wiki:
She spoke with a simpering, high-pitched voice that was girlish and breathless, and has her office decorated with kittens and other cute pink things. She had a wide, toad-like smile and would after say cruel things in a sugary, sweet voice.
The pink clothes & accessories, the kittens and the high-pitched voice are to depict girlishness.
According to Harry, she looked like someone's maiden aunt, and that her style of dressing would fit someone half her age.
We can say that the filmmakers wanted to show irony/contrast in the character.
She dresses & acts like a sweet maiden, but actually is a ferocious, unfair and cruel.
Sources: Dolores Umbridge < Harry Potter Wiki
A glance through official wikis and 'making of' sites does not appear to shed much light on this, other than the fact that she seems to dress like a favorite auntie.
Personally, I think it was a conscious decision on the part of the filmmakers to dress her this way, thus making her the living embodiment of 'a wolf in sheep's clothing'.
Sickly sweet pink on the outside, twisted and rotten on the inside, a perfect contrast.
Pink is to Umbridge as black is to Snape. Umbridge acted like a little girl in some ways, using it to hide her true self. Her gaudy pink visage was meant to make her seem respectable and unthreatening, helping in her political machinations and her actions as Hogwarts High Inquisitor. The pink walls of her office and the kitten plates, etc etc contributed to this.