I always thought code black was a bomb scare or similar, code blue is generally for imminent loss of life (heart attack, etc.). Different places will have their own systems though.
There really is no universal "guarded language" for these incidents. I have worked at many many music festivals, large scale public events and so on where there are huge gatherings of people. Security is top priority and the hidden work to prevent and minimise danger is truely mind boggling.
Each event I have worked at uses different guarded language that will usually not be at all obvious to the public. Tannoy announcements are even more cryptic. For example "Can the amber team leader please come to event control". That would indicate a stand by and listen to your radio or prepare to take your specific pre prescribed actions. "The amber team leader has arrived at event control" would be a stand down.
Guarded language is generally used more than individual words or codes at public events and large public sites as they generally stand out like a sore thumb. The public would dismiss a call for the amber team leader (as he could be in charge or replacing toilet rolls) but a "Code 1" or "Code black, red, yellow or green" is quite obvious and will get swan neckers swaning!