A prettier colour in comparison to the original. I can not find any reference to this shade. I have to conclude this is a changeling even though I have seen it offered without the overprint as a colour trial at a national stamp fair.
There is a slight defect in the bottom margin which would have affected its value. So somewhere along the line, the colour has been altered in an effort to enhance interest. It goes to show as a mug like me paid way above its intrinsic value.
Apparently, by using solvents like methyl ethylene ketone MEK or toluene, it is possible to selectively dissolve yellow and red pigments leaving behind the blue shade.
What puzzled me initially was that the stamp has almost full gum. This is because no water was involved and so the gum was left undisturbed.
Despite doubts to its authenticity, I have enjoyed looking at it and it has its space in my album. There are a couple of anomalies with these 2 stamps that readers may have observed
Update: There is a mint single 1924 2c postage due stamp in the Royal Collection in black and turquoise blue which has been classified as ? changeling ? colour trial. There is also mention of a pair of the same stamps by Agas in the Sarawak Journal. So I feel the jury is still out with regard to the status of this stamp.
References: Sarawak Journal vol 29 p 178/179, vol 35 p 17