you should use the red top for be sure that there won't be any problem.
btw i saw a lot of cosplayers with soft air weapons without the red top
during the convention there will be police and similar, but it's for general security (there are so many people, the city is totally full of cosplayer and not only them!)..
uhm a solution maybe could be to bring your gun with a removable red top and put it into your weapon if it's necessary for control :P