Gold Coast KGV 1sh Color Help

Three different Gold Coast KGV 1sh stamps.

The first one is wmk 3 (multiple crown and CA).
The second one is also wm3.
The third one is wmk 4 (multiple crown and script CA).
All are chalky paper.

I can find the first and third in the 2019 Scott Classic catalog. Scott #75 (black on green) and #90 (black on emerald). However, I cannot find a listing for the 2nd stamp.

Catalog lists for wmk 3:
75 black on green
75a black on emerald
75b black on blue green but then also says "ol back"
75c Die II

The second stamp is not Die II. It is not 75 or 75a. Must it be 75b? I just don't understand the description. The paper certainly is not blue green, but under a microscope I can see tiny blue flakes. On the 75, I can see green flakes. I don't understand what "ol back" means.

What am I missing?

This means "olive back"

That's actually the first thing I thought it might be, but it's not something I have seen in any other place in the catalog. That said, the back is the same color as the background on the front as far as I can tell. Neither look olive to me. The main design seems an olive brown or a gray brown with the GOLD COAST in black and the denomination tablet in black. Perhaps there is some other listing in the non-Scott catalogues.

Changeling.

Exposure to sun or chemical.

Not sure I'm buying the changeling here. Wouldn't the postmark or the black ink or the chalky surface have been affected?

UV light from the sun affects the colours, but not the chalk-surfacing. Also, whereas fugitive ink was a measure against cleaning of used stamps, cancellations would have to resist cleaning attempts. That the ink of the cancellations is not or less susceptible to fading should not come as a surprise.

It can only be light- or chemical-affected - the stamp simply doesn't exist in brown.