Can I get my recruit back after he has turned red?

Solution 1:

You have 6 recruits, and unlike previous games, they are unique and can't be changed. When a recruit is "injured" during a fight alongside Connor, or takes part of a failed mission, they will become red, and won't be able to come to Connor's aid in fights or go on mission, for a limited amount of time, after which, they will "heal" and return to normal.

They can't die, and will forever be with you. Also, losing a recruit to injury doesn't seem to have any implications on the recruit, so you will get them back just as you left them. Just wait a while, and you will get them back, all healed up and ready to kick some Templar a**.