] ]However, W. stated clearly he strongly dislike CB, with the result that Marianne changed her sentiment from feeling sorry for CB to also disliking CB. Is that right?

] I can't see anything that indicates that Marianne disliked Colonel Brandon before Willoughby started criticizing him.

] But Marianne never makes personal criticisms of Brandon's personality or character before Willoughby comes on the scene.

A bit earlier in Chapter 10, one of the first things that Elinor notices about Willoughby (other than how good-looking he is!) is the following:

Elinor saw nothing to censure in him but a propensity, in which he strongly resembled and peculiarly delighted her sister, of saying too much what he thought on every occasion, without attention to persons or circumstances. In hastily forming and giving his opinion of other people, in sacrificing general politeness to the enjoyment of undivided attention where his heart was engaged, and in slighting too easily the forms of worldly propriety, he displayed a want of caution which Elinor could not approve, in spite of all that he and Marianne could say in its support.

Marianne strongly approves of people who speak their mind openly, and when Willoughby gives his opinions of Brandon, she is eager to join in and agree with him. It's a show of support and approval for Willoughby's behaviour.

Marianne's declaration, which she adds to Willoughby's criticisms, is that Brandon "has neither genius, taste, nor spirit. That his understanding has no brilliancy,his feelings no ardour, and his voice no expression."

And, as Elinor says, Marianne has formed this impression 'on the strength of her own imagination'. What authority has Marianne to form any such opinion of Colonel Brandon when she has ever even had a conversation with him? What harm has Brandon done to her at this point and when has he ever been anything other than a perfect gentleman to her and everyone else? The only possible motive for Marianne to say such things is to impress Willoughby and show him how perfectly matched they are in 'every point'.