There are different kinds of knowledge here. One is an information-based knowledge, the kind Mary has with regard to science (which apparently is complete). The other is an experience-based knowledge that is incomplete. It doesn't matter what head-knowledge Mary has before she ventures out. Once she sees color for the first time, she will learn visually. She would also need to learn to associate names to the visual representation of color.
This is actually how I learned music. I was born with perfect pitch, the ability reproduce a specific tone without a reference point. When most people learn musical notation, they learn that the note on that specific space is 'A'. But in addition to learning the visual aspect of it, I learned that the note I heard was called 'A'. So every time I hear a note or chord, I know what to call it. This is exactly what Mary will learn when she experiences color for the first time.