This is an interesting read: Target Practice: Racism and Police Shootings Are No Game

Basically there is a computer shoot/no-shoot simulation and it seems that the general population is slightly more likely to shoot black people than white people. The simulation has people holding weapons (shoot targets) and things like cell phones and soda cans (no-shoots.) Apparently police officers do better than general public and exhibit less racial bias. The online version of the simulation can be found here

In such situations the actions of the subject are going to dictate responses far more than visual object recognition. If a subject acts in a manner consistent with being a threat he is more likely to be shot. I have been to some bad scenario training where somebody quick-drew a radio and pointed it like a gun (he was shot before he finished his draw stroke) and the student that did the shooting got (wrongfully) berated by the instructor. The person with the radio was a threat in the scenario, and his actions were consistent with drawing a weapon. Apparently the instructor would have waited for a muzzle flash.

When facing a law enforcement officer with a drawn gun I would suggest that is unwise to quickdraw your cell phone when you have been told not to move.

Consider the Diallo Shooting:

The officers were in plain clothes. The officers claimed that they loudly identified themselves as NYPD officers and that Diallo ran up the outside steps toward his apartment house doorway at their approach, ignoring their orders to stop and "show his hands." As the suspect reached into his jacket, Carroll believed Diallo was drawing a firearm and yelled "Gun!" to alert his colleagues. The officers opened fire on Diallo and during the burst McMellon fell down the steps, appearing to be shot. The four officers fired forty-one shots, hitting Diallo nineteen times. Investigation found no weapons on Diallo's body; the item he had pulled out of his jacket was not a gun, but a wallet.

The officers involved in the shooting were indicted for second degree murder but were acquitted of all charges. One could argue that each officer that fired needed to get independent confirmation of a threat, but it would be pretty hard to stand around flat footed when someone on the team yelled 'gun', the shooting started, and another officer fell to the ground.