The first thing Lady Capulet does is phrase the news of Juliet's upcoming marriage in as positive a light as possible. She does so by explaining to Juliet that her father made the decision to "put thee from thy heaviness," meaning to distract her from her grief over Tybalt (III.v.111). In doing so, Lady Capulet also praises Juliet's father, calling him a "careful father," meaning "mindful" (110; Random House Dictionary). She is saying in this phrase that her father is being mindful of her grief and thoughtful enough to find a solution. She also phrases the event in as positive a light as possible by referring to the news as "joyful tidings" and to the day as a "sudden day of joy" (107; 112).
However, when Juliet refuses, Lady Capulet's behavior becomes darker. She washes her hands of Juliet, wanting nothing further to do with her. We first see Lady Capulet wash her hands of Juliet when she tells Juliet to tell her father herself that she refuses. We further see her washing her hands of Juliet when she declares, "I would the fool were married to her grave!," which is not only a very dark reaction to Juliet's refusal, it's very unmotherly and excessive (143). We further see her washing her hands of Juliet in her final lines of this scene when she says, "Talk not to me, for I'll not speak a word. / Do as thou wilt, for I have done with thee" (211-12).
In short, while Lady Capulet's behavior in this scene is at first positive and even shows concern for Juliet, her behavior becomes excessively dark and unloving when Juliet refuses to do her parents' will.