I was at a gunshow today and I saw complete FN bolts w/o carriers for 50bucks. The bolt in my Colt has seen better days and it will be soon time for a new one. Would the FN bolt work in that gun no problem? The gun I`m talking and is the 16 inch carbine

You should be OK changing out the bolt, but it would be worth checking the headspace with the new bolt before firing it. People have been switching them around for years without checking headspace though, and I've never heard of anyone having a problem (though I'm sure some have).
The one thing I would mention though, FN does not sell parts on the civilian market, so if someone is selling "FN" bolts, I'd be careful. They may just be surplus, or they MAY be rejects or just off-brand parts being sold AS FN parts. You can buy a brand new Bushmaster bolt, direct from them, for $49.95 any time and use the $5 admission fee saved from the gun show and use that for shipping.

www.bushmaster.com/shopping/uppers/8448509.asp