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:
The pink clothes & accessories, the kittens and the high-pitched voice are to depict girlishness.
We can say that the filmakers wanted to show irony/contrast in the character.
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.