Well the entire point of Spirit is they aren't going to bust your nuts over the color of your suit or tie. We hired a guy wearing a yellow suit.

This isn't Delta where every applicant shows up in the exact 2 button navy suit with red tie, simply because the interview consultant told them this is the interview uniform.

Dress like a grown up, and its not an issue. Most interviews usually begin with the interviewers telling people to take their suit jackets off and relax; this isnt a test, its how things are.

If you really want to "look the part," show up in a NorthFace and they will know you are Spirit material!