The different colored 'Rings" around primers
serve a combination of purposes. The color
codes indicate what type of projectile the round has,
Tracer, Incendiary, Armor piercing etc.
Plus, the colored laquer acts as a sealer. There is normally
a ring around the bullet at the case mouth also.
I have seen red, black, green, and a dark purple.
the dark purple seems to be the most common.
It should be easy to tell the difference between
corrosion damage and the primer Anulus (Laquer).
And yes, the powder does deteriorate with age.
but normally not enough to tell in shooting it.
A chronograph will see it though. I have fired
.303 Brit. rounds with headstamp dates of 1909
(In Arabic) and the primer was the fault of the misfires,
not the powder (Cordite). They still shot with authority when they
did go off!
The Exception to this rule is old blackpowder rounds
(Which do show up from time to time)
Black powder actually becomes more powerful
(and unstable) with age!!!