In my experience of IB either brown or black is an acceptable choice.
Black tends to be preferred for client meetings and is more popular with over 40s.
It is always the more conservative choice.
I have a friend who works for Bloomberg (a more relaxed office environment) and he was terrified of 'deviating'.
He finally bought a pair of dark slate EG wingtips (barley distinguishable from black, under all but the brightest of lights) and was relieved to find that nobody cared in the slightest. As a consequence he has now bought a pair of oxblood and a pair of dark brown captoes. These have also escaped criticism.
Perhaps it is more subjective now and a question of status (together with inherent confidence). I know a few senior city folk who wear anything but black shoes, they are not wage earners however.