There are always exceptions to the rule. However, in about 95% of interviews, you need to wear a suit.
About a year ago I was doing an interview for a magazine story on a local newscaster here in PHX who races Mini Sprint cars on a dirt track. Now, in typical journalistic style, I would have worn at the very least a dress to most of my interviews. However, the mud out at the track is clay; very sticky and very dirty. My attire? Jeans, tennis shoes (old ones) and nothing that was white!

It did not ruin my credibility in this situation because wearing anything else would have garnered a few chuckles from the guys and gals out there.
Many years ago, when I was conducting a seminar at a local medical trade school, one of the job seekers insisted that they could wear "scrubs" to the interview because that was their normal dress. I disagreed with her on this point, as an employer wants to make sure that you can "clean up" appropriately, in case they need to send you to a seminar or a training session. Once she understood that, she realized the value of dressing up.
I know that Kit Darby says blue suit, white shirt and red tie. But, if someone showed up in a black suit with a blue tie, that would not get them bounced from the interview. What I always wanted to know was that a candidate would take the time to look nice, with a properly fitted suit, shined shoes, matching socks and their hair and nails clean. It was an indication of what they would look like when they showed up for work.
Kathy