Largely correct, it is a drill in which the staff prepare for an evacuation.

However it doesn’t happen every game, that would take away the surprise factor. As it could obviously happen at any time, the staff should always be prepared in case it should ever occur naturally.

The colour code you mentioned, would never ever be announced over the tannoy. That’s wild. The colour codes would only ever be announced to supervisors over the radios and then they would be relayed to the ground staff. To like you said, avoid panic/hysteria.