I agree that depending on cut and model you can move between two sizes (I am usually 44L but can fit into some 42L), but frankly I just use size as the first filter when looking for stuff to thrift. ...If all but one measurement is OK, I still pass (the exemption being sleeve length which is easy to alter, assuming there is enough material to lengthen it (as I always need to let the sleeves out)).
No they are sort of medium grey, they just look charcoal in that picture.
Have you ever been able to size down from a 46L? I'm also a 44L and recently found a 46L Samuelsohn SC for 40% of retail that fit for length and sleeves. If the shoulders weren't absolutely perfect they were very close. The jacket is being slimmed through the body and I can pick it up in a week. I've been told that Sameulsohn is a good brand when it comes to alterations but I dunno as the guy who told me that was a salesman.
In reality I'm probably a 43L (in-between 42L and 44L) so 46L would in all likelihood be too large for me regardless of cut and model. I have a JP Tilford (which I believe is made by Samuelsohn) jacket in 44L that is also just about perfect in the shoulders but that also could use some slimming across the waist.