Left - normal paper (lighter blue). Middle - scarce thin paper (darker blue). Right - very scarce aniline ink (blueish green).
Aniline ink is a water-soluble and fugitive ink, with a dye base that runs when wetted, and fluoresces under an ultraviolet light. The colour will migrate through to a marked degree on the back. Aniline is not limited to one colour, it has been used in the KGV 1d (red).
This particular stamp was not popular with collectors of used stamps as the ink was deliberately made to leech when wet and ruin the stamp preventing it from being re-used.
In mint unhinged condition the aniline shade in my opinion is stunning.

 
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