Active Mission?

I tried a mission and had a lot of difficulty getting killed and all and decided to postpone it and do something else. The original mission is now listed in red (I understand that if I go back to the mission it will be as I left it with any baddies I killed still gone) but it seems to be saying that if I don't complete that mission I can't do any others. This red status appears to go away after a certain time, or is it a restart of the game? Not sure about the mechanism here. Thanx.

Red missions means its the active mission. You can only have one active mission at a time. If you abandon that active mission, the mission will reset and the text will be yellow after that. This can happen from logging off, joining a team with another active mission, or activating another mission yourself.

EDIT: You can activate another mission yourself by simply entering a new mission on your list or (I forget if this is there if not a team, so I could be wrong) on the mission tab, there's a "Select Mission" button that you can use to set another mission if you have multiple.

Did you, by any chance, talk to a Field Analyst and change your Reputation? Reputation = mission difficulty setting. Heroic is the lowest setting. COV side, Villainous is the lowest setting.

One thing to note: Your mobs will respawn after 15min or so of being outside of the mission. So, if you're thinking you can keep it up and go back to it in an hour and pick up where you left of, you'll be a bit surprised when you find everything respawned.