New Zealand Definitive Unknown Colour

I recently bought a collection of New Zealand stamps and found this brown 2d definitive, a stamp that is meant to be orange according to all sources I've used. I've put the supposed correct colour to the right of the stamp.

I haven't actually been able to find any identification for the brown stamp whatsoever; is there an issue of the definitive that I'm missing?

Thank you for all of your help.

Colour changeling.
Engraved
Pigment in the ink has oxidised (Chemical or light)

The 1 shilling Kea Scott 61 will change colour similarly.

To expand a bit on what Rod has correctly posted.
Stamps, especially stamps with orange hues, can change colors or darkened with age or from contact with air or sunlight. It is often assumed that oxygen is the culprit. In most cases it is actually the sulfur in the air, albeit in minute quantities, that combines with the metallic component, often lead, in the colored pigment. Nearly all metallic sulfides are black and this is what "darkens" the stamp. The sulphurization is reversible with the application of hydrogen peroxide, which replaces an oxygen atom with the offending sulfur atom, restoring the color.
Don
APS #094826

Thank you ever so much for your help - I had some knowledge of this with Victorian Golden Jubilee stamps, but I had no idea the change could be so prominent!

I must admit, though, I thought for a couple of minutes yesterday that I'd found an colour error - it's slightly disappointing that it's not!