Mistook new faculty member to be a student, how big a faux pas is it?
I'm a female professor in my late 20s and I'm pretty sure I look even younger than I am. I've been mistaken for a student a bunch of times and it's always been an honest and totally understandable mistake. I'm often assumed to be a grad student; heck, I discovered in front of my mirror three days ago that wearing the school logo T-shirt I somehow acquired is inadvisable because it makes me look nearly indistinguishable from an undergrad.

I know doing this is embarrassing for the person who messed up, but looking at me and concluding that I'm very likely a grad student is reasonable and something I know is going to happen unless I'm wearing a fancy-schmancy suit. Heh! I've even done it myself. One time when I was meeting a (male) colleague, each of us mistook the other for a grad student; we've been teasing each other about it since. The only circumstances under which this would bug me would be if the person doing it were trying to be patronizing, or covered it up in a way that introduced some creepiness.

But yeah. I'm accustomed to it and I know my status surprises people. When someone asks if I'm a student, I laugh it off and say, "I am a very young faculty member." Most of the time thereafter I forget it happened. You did everything you needed to in order to make amends; I'd say put the guilt aside and move forward with a professional relationship with your colleague.