The Many Tones Of Lincoln Cent Proofs

Given my experience recently with adding Lincoln Cent proofs to my collection, I figured I would make a separate topic about this.

It seems to me that many Lincoln cents come in various "tones," in color. It's kind of frustrating to that degree for me personally, because I am trying to keep everything uniform.

But, what exactly causes this? Why does one proof, even around similar years, look vastly different in color/tone? Does the "deep cam" have an affect on such things?

Here's a few examples of what I am talking about -


A bit of a lower quality pic... But you can tell where the 76 proof really stands out. The 74-S and 75-S are also proofs, but match better with the rest of the coins.