I figured it was a woolen beanie edged red.
The one in the artwork is a hat worn over the red strip of cloth they were issued as a sign to distinguish them from Zulus.
If you look at this photo, particularly the fellow one in from the left...
...Then I don't think there is a whole lot wrong with the sculpt. Hard to say if that particular example is a woolly hat or some kind of shapeless felt hat. Doesn't really make any difference which it is, though.