Oxidation on Brass?

[Sugacookie]

Pardon the dumb question. I have some New (But Old) never fired Rounds of .223 Remington. Ive noticed that on the Top of the Brass Shell below the bullet the Brass is oxidizing. Rainbow effect.

Was wondering why this is occurring also if it is bad for the brass itself?
Thanks!

[Spdjunkie]

I can only assume without pics to go by, but sounds like annealed brass & not oxidation?

If you google annealed brass cases under images it may answer your question.
Hope this helps.

[Bug Splat]

Yup, sounds like annealing. I hear they can get darker and more noticeable over time but I've never kept brass I've annealed long enough to find out .