US Philippines Envelope Colour Variations

An extension of my US collection is the US in Philippines. A very nice gentleman at my stamp club is always offering me stamps from this country, though they are mostly modern. In his offering the other night these cut prestamped envelopes from the possession were included. I don't really collect the prestamped stuff, but, as usual, accumulate some along the way, and these I don't often see.
What interests me is the colour variation. Scotts Specialized lists these as E1 issued in 1908, but only describe them as green (they do get into the variation of paper colour, but these are both white paper).
Don't know if you can make out from the scan, but they are very definitely two different greens. The left example is deep yellow green while it's partner is more olive in tone.
My long winded question is, are variations in tones on these common? So common that Scott's doesn't bother to list them? Or could it be some sort of oxidation?
They are by no means valuable, even less so with their trim job. But they make me curious.
Thanks

In addition to the Rizal Type E1 envelope, the same embossed design exists on the wrapper W4, which was printed on manila paper. When they are trimmed so close, it's almost impossible to be sure about surface colors, but if one looks "manila" on the back, then that's a possible explanation for the variation in color.

The other point is, some of the E1's are 100 years old; in a hot, damp country like the Philippines, you could find a hundred variations in color, all due to environmental conditions, not printing.

I don't have a catalogue in front of me at the moment but on first glance I'd guess that they are from different dies. The forehead hairlines seem a bit different and the "U-S-A" seem to appear different as well.

Again just an educated guess (while I'm here at work without a catalogue - shhh, don't tell anyone)

Differences in the "DS" of Islands are very plain, I agree.