For player colours, see this thread:
For aircraft colours on the minimap (pilot’s perspective):
- white: your aircraft
- blue: other aircraft
- orange: alert: other aircraft that’s in close proximity to you (over three miles, but possible traffic conflict)
- red: high alert: other aircraft potentially in collision range (I think it’s either <3 or <1 NM distance and less than 1000ft altitude clearance)
It’s a basic emulation of a TCAS system.
If they disappeared - depending on which server you were flying on, it could be any of the following:
- they quit the app
- app crashed
- they were reported by users and the system ghosted them
- they racked up a high number of violations and the system ghosted them (this is automatic)
- the ATC ghosted them for inappropriate behaviour (advanced server only)