Dress code for interviews is a pretty basic affair. Wear something respectful and formal. Make the interviewer see that you take college admissions seriously.
When I email candidates, I often tell them that dressing casually is fine if it is more convenient. One of the main reasons I say this is that I ask candidates to meet me early on in the day, and if they are coming straight from school, I don't want them to feel obligated to show up to class looking like they're going to a wedding.
Most candidates don't take me up on this, and still arrive decked out in business suits. I appreciate that they are so respectful and take the interview seriously.
What to do if your interview (like me) gives you the option of casual dress? Or if they don't say anything at all?
It doesn't matter where you are meeting - an office, a house, a coffee, or a circus... but if you don't get any guidance, dress like you would to a job interview, a debate tournament or formal affair. For men, this means a suit. For women, slacks and a white shirt or sweater is nice if you have it, if you don't own anything quite that formal then muted colors and conservative attire is fine.
If you do have the option of dressing more casually, try to stay within the more conservative edge of that range. Think about it as something you would wear to church. If in doubt, dress more formally rather than less. It can't lose you any points, and your interviewer will appreciate it, even if she doesn't say so.