I recently got a colt 1911 pistol for my birthday. (Sometimes younger brothers can be pretty cool). Anyway, I downloaded the owner's manual from Colt and disassembled it following the instructions as best as I could. However, after I put it back together, (again following the instructions) I noticed that when the slide is locked back, the barrel moves side to side ever so slightly, it's not much, but I can tell it does move. Is this normal or did I screw something up?

Normal.....

Slight play in the barrel when the slide is locked back is perfectly normal.

Yup Normal.

Thanky, I feel a heck of a lot better now

As the other folks have said, it is normal. A standard M1911 has a lot of built-in slop (large tolerances) to make sure it will function under adverse conditions, like when it is dirty, etc.

The match-grade versions are generally tighter, but you might still get some side-to-side motion of the barrel when the slide is locked back......

What's important is that it stays put while in battery, with the slide forward. I'm sure it's fine.

Congrats on the new gun.