Put a new night sight on Glock 19 after work yesterday and did not use any thread locker / locktite.

Will it be an issue?  Should I re-install the sight with lock-tite? - if so, red or blue?

I use red and only because you can't really get a good bite on that little screw. It's so easy to remove and reinstall it wouldn't be much trouble to Loctite it. I have OCD so I would have to.

Good call; thanks Bradd; I will do it right.

Murphy's law: my FS would fly off the one time I really need it.

I'd take it back off and use thread locker. I put night sights on my G19 and didn't have any thread locker. Took it out to the range right after putting them on because I wanted to make sure they were zeroed and I had to carry it for work the next day. I'd have to strip the gun after every magazine and tighten the screw back up. Even though I snugged it down, it kept loosening up. If I hadn't kept stripping it and tightening up the screw, the sight would have flown off. After my range trip, I stopped and got some thread locker (LocTite, the weakest one, I can't remember the color). No problems since using the thread locker.

Bub75

Loc tote is the answer