Hi Stuart
You didn't say which vehicle you are working on but from the metric size I'm going to assume that it is a vehicle that uses metric bolts. Only mention this as it is a pain to do repair work which is a mix of metric and inch fractional.
Now to your question on 'My question is what is the advantage and disadvantage of the finish such as Plain, Yellow Zinc and Zinc in these locations?"
The biggest problem I found with most modern bolts is getting the paint to stick to them if you are painting them. Plain bolts with no protective coating other than the oil base coating need to be cleaned to get a good bond with paint.
Yellow Zinc and Zinc paint does not bond well to at all with out etching the bolts. The metal etch that you use to prep bare steel before painting will etch or out right remove the Zinc coating. The paint will stick better but the bolts and hardware will rust if not well painted.
On things like bumper bolts I stand the bolts in a shallow tray of the metal prep so just the heads are etched. Leaving the threads still coated.
My experience is that the plain zinc and shinny zinc is generally used on standard grade bolts while the yellow zinc is usually used on grade 8 bolts. Not being impressed with much of the standard grade bolts hardware, coming out of China too many broken bolts and stripped threads. Deffinitly not the same quality of bolts CMPs were built with 60+ years ago. I have gone over to using all grade 8 bolts and hardware. Buying most of it in bag lots of 25 to 100 from Fastenal
http://www.fastenal.com/web/home they sell in quantity but if you are going to hardware store or auto parts to buy 10 or 15 regular grade you can probably by 25 or 50 grade 8 for nearly the same cost.
Fastenal of course has all the metric and inch dimensional bolts from their web site it looks like there are quite a few outlets in Canada.
Cheers Phil