The following has already been brought up in a comment, though from a user who already vanished, and might just be a little addition to the other interesting answers, but it definitely deserves its own answer.
There is actually this excellent article about analysing the clothing colors of many of the major characters in Breaking Bad, which also presents a very neat color timeline for each character.
And in particular about Marie (who pretty much sticks out with her prevalence of purple) it has to say
What makes Marie noteworthy isn’t that she wears all purple, all the time. Rather, it’s the very rare occasions when she’s not wearing purple that practically scream at the viewer. For instance, she wears black when her kleptomania flares up. She wears black again when she’s in protective custody of the DEA after the threat on Hank’s life at the end of Season 4. And then finally, she turns yellow just before Hank makes his massive discovery at the end of the first half of Season 5.
(The last fact is also relevant when seen in connection to this related answer about the remote controlled car, which also presents many thoughts about the usage of color in the show, especially about yellow being a signal of danger.)