Silvers can be a real pain to blend and often there's more than one formula
for a particular shade. Some bodyshops who don't have their own mixing facilities, will just order paint to the code number and, if there are different formulations, just stick with 'No. 1'. Generally speaking they won't paint edge-to-edge (that is the whole panel) e.g. if it's a wing, then they would spray the wing and maybe blend into the door, or if it's a bonnet they'd do the bonnet and upper edge of the wings. If it's a couple of doors then they'd probably spray the whole side.
A lot of colours 'halo' but silver tends to be the worst, therefore a local repair is
a pain in the backside, though some silvers tend to blend in better than others.
A blend showing under street lights for example, doesn't mean a 'bad' blend.
Often this is simply due to using a different paint to the manufacturer. There are different pigmentations in different manufacturers tinters and they'll show up
differently under different lighting conditions.
Metallics/pearls in general can be difficult to blend and it is mainly because
of the metal flakes/mica particles falling differently to how the original paint was applied. Realistically, it's virtually impossible to spray at the same distance, pressure etc. that was applied in the factory.
Reds, I've not (touch wood) had a problem with.
Local repairs can work depending on which area of the car they are applied to.
Low down you can often get away with it and on contours such as bumper corners, then you don't get a problem. Large flat areas will show up more.
Overall a large contributory factor is also who is pulling the trigger on the spraygun
and even the best have off days and finally another thing to bear in mind is
that everybody sees colour differently, so what is perfectly good to one person
can stick out like a sore thumb to another.
:)