Picked up a pretty complete Colt M16A1 parts kit (though with what looks to be a GM barrel, marked simply "5.56 NATO 1/12" below the handguards) for about $725 shipped. Looks to have everything but the lower receiver and a trigger guard.

Selected a Brownell's BRN-16A1 lower in gray for the lower receiver, but I'm a little stuck on where to source the remaining parts: a semi-auto AR15 FCG, semi-auto AR15 safety selector, and trigger guard.

The FCG shouldn't be a big deal (I have a few mil-spec FCGs kicking about and they aren't exactly hard to source anyway), but I had some questions regarding the safety selector: were refurbished or otherwise very late production M16A1 rifles supplied with black "tick mark" safety selectors, and black trigger guards?

I don't mind building this parts kit into a decent representation of a refurbished/late production M16A1, but I'd like to get some input (ideally with pictures if available) to see whether this path would be authentic, or if I should just keep scrounging for a gray "no tick" safety selector and gray trigger guard. (If you happen to have those parts for sale or know where one can find them, that would be much appreciated.)

The only refurb A1 (and XM) lowers I saw in service had been converted to A2 rifles, so all of the lower parts were black in finish and the selectors had the tick mark. For new grey parts, I think AR15Sport is the place to go. I have one of their complete LPKs ready for my next build, but have used the following individual components in others.

https://ar15sport.com/product/repro-alum-mag-catch-button-in-xm-grey-retro-magbut/

https://ar15sport.com/product/repro-milspec-alum-trigger-guard-finished-in-xm-grey-retro-trgguard/

https://ar15sport.com/product/repro-a1-style-no-tic-two-position-selector-ret-notic/

Thanks! Unfortunately only the selector looks to be in stock on that site - any ideas on where to get a gray trigger guard?

You should email them at AR15Sport and ask when they'll have it in stock, they've always been quick to respond when I've emailed them.

+1 on ar15sport. Since the middle of January I've made three orders and have been excessively happy with all of them. I bought things like a non-stick selector, bolt catch, trigger and hammer pins, mag button, and trigger guard. Everything is just right. Their three-pronged are the nicest repros I have seen yet. They are now my go-to when it comes to retro stuff.

The anodizing on their aluminum parts matches NDS very nicely.