If you pull your trigger to the first detent (which selects the gun) a side effect of this is the HUD symbology changes to have a circle over your target. It works well for doing a quick check of where the target is, then release and the symbology automatically snaps back to the normal lock / intercept guidance circle.
This seems to be pretty consistent. If you switch your hud from day to reticle, and then try to change it back to day, the color does not change back to green. I'm not certain if this is a bug behavior or not.