A lot of it comes down to the individual details.
For example, if the CR blue on your lift rings doesn't perfectly match the CR blue on the hood, it'll be tough to tell because of the lack of direct comparison (they'll pick up light differently, etc...) and lack of a "field" to compare.
But say you're putting on a cab AC unit that is supposed to be the same color as the unit. If it doesn't match decently, it will probably stick out more.
Light weathering (which you were going to do anyway, right?) can hide some minor differences (it's amazing what a flat black wash has done for some of my stuff), but sometimes you can still tell.
And as far as painting before installation vs post installation, again, it's really a case by case thing. I usually install first, then paint, but that's just because of the parts I'm installing, and their lack of good "handles" to hold while pre-painting. The downside is that I just have to be a bit more careful while painting.