James-Lewis joined the club from Southend but had yet to break into the first-team and, with his contract expiring at the end of the month, the club decided not to offer him a renewal.
Chelsea midfielder Nkumu had not done enough to force his way into manager Joe Dunne's plans after arriving on a short-term loan deal.
Dunne told Colchester's official website: ""As I've said before, with the lads in the Under 21s I need to look at how their progression is going in getting towards the first team and Merrick was some distance from that side.
"Letting him go now gives him a chance to find a club in the January window.
"Archie struggled with injuries during his time here. He's technically a good player, given his Chelsea background, but he found it difficult here.
"He's been a good lad, who was willing to do well, but it's not kicked off with us and I need to move on.
"I made the decision that he goes back to Chelsea and I will now look to recruit elsewhere."