In Breaking Bad, the last 5-6 episodes of season 5, Marie who wore purple throughout the entire series, starts wearing black. This happened after she learns the reality of Walter White. Why this switch in her clothing?
Black symbolises death, darkness, evil.
Marie is not evil. Being the sister-in-law of Walter white aka Heisenberg and wife of a DEA agent she realises the gravity of the situation they are in i.e,how close they(or her husband) are to death.
She thereby gets out of her normal self.
...After learning that Walt is a meth kingpin, Marie begins to demonstrate a darker side. She tells Hank that he must apprehend Walt at all costs and even says straight to Walt's face that he should just kill himself in order to free the family from the burden of his criminal activities. She even seems to have fantasies of killing Walt herself as implied during a conversation with her therapist in which she discusses her fantasy of poisoning Walt (under the guise of a "family friend") and claims that while she will hurt nobody, it feels good to think about it. It is evident that Marie deeply despises Walt for the atrocities he has committed and what he has done to the family and clearly wants him dead or captured at all costs. Source
The change in her nature is depicted through the change in colour she wears.