Why The Difference? (Or, 1 Of These Things Is Not Like The Other!)

I'm scanning these CDV's because one of these R6C's is distinclty different from the rest. I know scans and photos are lousy for color differences but hopefully you'll be able to see it and help me out. Why is it so much different? I don't think it's green paper (the paper doesn't have any sort of green tone to it, and I don't have a uv light to see if it glows). Big scan is at 300dpi, individuals are at 1200dpi.

I'm also including a high res scan of a stamp that has been cut off at the bottom to see if any of you experts can tell me what it is? A stretch, I think.

RevenueCollector I have reread your post from 2013 so I especially interested in what you think!!

Here's my confusion: the R6C that is fairly well centered inside a scallop edged frame (Wharton & Noftsinger photographers on the big image) is very much "browner" or "brown red" colored than the rest of the bank check stamps. Why? What happened? Please share your knowledge!!!
Thanks in advance!!
Dave

The stamp in question is not a different color (at least when it was printed), but has been changed due to oxidation (actually, exposure to sulfur, likely from the cardboard used to make the photo or the envelope in which it was held).

The second stamp is a very dry print. Otherwise they are essentially all the same.