It is entirely possible, in your interview process, that every person you meet is not wearing a suit; some may not have shaved. That doesn't make it wise for you to turn up in 'smart casual.' How you look is not a big deal for quants, you're being paid to think. However, some people do get remembered for the wrong reason, and it can undermine your application a little. You should feel comfortable, and if that means a bit of perfume or good cufflinks then that's fine, but see below.
Neatness is good
More important than colour of cloth or design of tie, is the general impression of being in control of how you look. This means wearing it well, and being ordered in your appearance. It is worth checking this before you go into the bank.
Black is the new black. White is nice for shirts and for no other visible item of clothing. Shoes should be clean and preferably black for men, and muted tones for women. A particular issue for women is the poor workmanship in most of their shoes. Do not attempt to walk long distances in new shoes that hurt your feet so badly they bleed (we know one person who stained the carpet with her blood). Make sure your clothes fit - badly fitting clothes do not look presentable and if your trousers are too tight you (and everyone else) will find this distracts from the matter at hand. There are some complexions that are generally complemented by certain colours, and apparently in some circles 'brown' is seen as a colour for your clothing. It is not; it merely says things about you that are never said to your face.
Dark blue is good as well.
Ties are best boring, novelty is bad.
Another reason for white shirts is that they don't show sweat, some colours do this terribly and it's not the image you want to project. A good shirt doesn't crease badly in wear.
This will never help you get a job, no matter how expensive or fashionable. Thus if you have any doubt at all, don't wear it. If you're female and you have some brooch or bracelet, that's fine, but there's no upside for a man at all in bling. Cufflinks of course are fine, as long as they are not 'novelty' - you have no idea as to the sense of humour your interviewer may have; he may not have one at all. Some banking people spend quite appalling amounts on their watches, so don't even try to compete.
Perfume and aftershave
Feel free to smell nice, but make sure that it's not too strong.
The following is for women. If you re a male reader, you really should not be reading this paragraph and we are rather concerned that you are. Unless you really never wear make-up, a small amount is a good idea. Again, this gives the impression that you are making an effort and will possibly counter the deadening effect of all the monochrome clothing you are wearing. It should be discreet (i.e. no bright colours) and presentable rather than intending to make you look prettier. There are jobs that you can obtain by being attractive, but they are rarely fun and never intellectually rewarding. Any make-up should always be well applied - if you can't get eyeliner on straight, don t put it on, and never wear nail polish if there is any chance it will chip before the interview.
What People Get Wrong
Zeroth law of holes
When you find yourself in a hole, stop digging. You will be asked questions for which you can t think of any answer at all. Some interviewers make the questions harder until that point is reached. The trick is to cut your losses. With any luck they will just move on, unless it s a critical topic. Of course if it s critical then it s game over anyway. What you must avoid is wasting time wandering like the lost spirit of ignorance over a vast formless expanse of your incompetence. A good response is to look them in the eye after a little thought, then simply say 'Don t know, sorry.
The exception to this are the 'all the tea in China questions where you are asked to estimate some quantity like the number of bull testicles consumed by McDonald s customers per year. You aren't expected to know the answer to these, indeed knowing it would seem rather strange. They want to see how well you can estimate an unknown quantity and how you think.
But the biggest hole catches people who get very nervous when things go wrong. This is about the most negative personality defect you might have in a bank. When you realize you've said something dumb, stop, say something like 'let me think about that for a second, and correct yourself. Make the pause work for you. Think the answer through, and show that you are capable of recovering. Remember that no one can talk about things at the edge of their competence for 4-5 hours without saying something silly. You don t have to be defect free, but self-knowledge and recovery will score you vital points.