Soviet Union Errors?

I have 3 pairs of items that appear to be errors.
Left image is as described in Scotts; right appears different.
Scott's standard does not identify color omission/differences for these.
If recognized errors, any thought on value?
Your help is greatly appreciated.

In the top pair, Scott's 1132 color is dark blue and dark brown.
Item on right is only dark brown (dark blue is not present.)

Any one have some insight on these?

Natural or chemical discolouration ?

Top right shows no signs of exposure to anything. Gum is in tact and there is no trace of blue on the face. Both are "crips."
Second row are both mint with gum. Same perf, but clearly different colors.
Third row, right, possibly exposed to a chemical (peroxide?) and somewhat "limp."

The variation in perforation quality on the first pair suggests a difference also.

The first pair (the 800th anniversary of Moscow) had at leat two printings in different years (first in 1947 and then in the 1950s), with a number of differences between them.

The second pair represents a definitive issue with multiple massive print runs during the period of its use, and it's no wonder that different shades of the same stamp exist.

The third pair is definitely a result of "human" intervention

Thanks to all.