What should I wear for a portrait shooting?
As so often, the answer is: It depends! And that is quite true, because it very much depends on the purpose of the shooting.
If you want to have the unbiased attention of the viewer directed to the face, there is a very simple choice. It is either a white or a black t-shirt. Weather it should be black or white depends on the background. I find it more interesting to use the same color t-shirt as for the back ground. By doing that there is the ultimate attention to the face and no distraction by anything else. I always suggest to my clients not to choose those cheap full cotton non-ironed ones in XXL but rather those which are a little tighter and wrinkle free because they are either ironed well or the nature of the fabric already allows a wrinkle free fit. Those best blend with the back ground and pronounce the body silhouette a little bit, which is typically nicer.
In case of a fashion shooting or anything like cosplay or so, any dress is possible, as long as it supports the idea of the photo. Here your face is in a balance with your outfit and sometimes even your outfit is more important than your face.
For business related portraits adequate clothes are mandatory. They should suit your position. Don’t be too overdressed, a little bit is ok. You do not want to shock your new employer or client, you want to present yourself in an appropriate way. Again, by doing so, you do not distract the viewer’s attention. You keep it, where it should be – on you!