Indochina 27 Color Question

Hi,
I've been putting in my album a huge French Indochina collection I recently purchased. My Scott catalog says 27 is deep green, of which I have 5 stamps. However, I have a blue shade of the same stamp. This is not listed in Scott. I would appreciate any help from my fellow Indochina collectors. Thanks

Could very well be a changeling


Peter

It is not unheard pf that a green stamp from that era turns blue.
But you, probably, require a Maury or Yvert catalogue to rule out the possibility of a blue stamp.

Look at the colour distortion near the PO of Postes, it's a changeling IMO.

Additive colour mixing: blue+yellow=green. So the printer's probably mixed up their ink using a base of, say, Prussian blue with a yellow pigment or dye, and the yellow component degrades when exposed to light. If the stamps were locally printed the yellow could be something weird, like Indian yellow, which was made from cow urine.

(edit:missing tag)

Thank you for your educational responses. What strikes me is how blue this stamp is compared to the others.
I googled Indian yellow, as mentioned by Flightle Bee and it was made from concentrated cow urine, of cows fed a diet of mango leaves in India. This was outlawed in 1908. Van Gogh's The Starry Night is said to have the sun painted with this Indian yellow. And Indian yellow is known to fade in the sunlight.
Interesting...