I think Johns card shows us the answer.
There are a few different ways to make red ink, the common one is with Cochineal, and another common one includes iron oxide.
Neither one is exactly red, although cochineal red is quite close.
There's a good picture towards the bottom of this page of the raw pigment.
http://www.webexhibits.org/pigments/...e/carmine.html
The ink suppliers typically supply a few basic colors and the press operator mixes the ink as needed according to a recepie. The brighter red would probably be a basic red lightened with some white and maybe a touch of yellow. Add black and maybe a bit of blue and you get a darker shade closer to maroon.
Another thing that happens to some inks is that one of the components can oxidize which changes the color. It's very common on orange stamps from the 1920's which can turn brownish.
Red is also prone to changes from light.
Where Johns card shows a mix of a darker shade and a lighter one, I'd think that the ink on a few has oxidized and darkened.
It's also possible the darker ones were simply mixed wrong.
Steve B