Not representing Zulu Ibutho's, the NNC shields apparently had no "pattern," though I have seen them black with white straps. This does conform with the Zulu practice of starting newly raised, young formations with all black shields, implying their inexperience.
But, again since that itself follows a fairly formal system, it may not apply.
Given the inherent woes of the NNC (not least their miserable volunteer white leadership, and the general contempt with which they were treated) I'd encourage painting the shields in every pattern. So a roughly even mix of "red/white," black, black/white, white/black, etc, should give them a very mixed bag look, again befitting the variety of tribes from which the recruits were drawn.
After all, the hides would have come from the same widely dispersed cattle stocks from which the Zulus made their own shields (but somewhat organized by color/pattern).
Depending on which campaign your figures are sculpted to represent, just paint the universal red rag around their heads, or paint their khaki uniform bits.
In a perfect world, your NNC should have white officers who look like drunks and scoundrels, while the rankers should look as individualistic as possible, within the confines of such "uniform" as they have, and no two shields alike. And if you can avoid using only a single pose, mix them up as much as you can.
Oh, and put your rifle armed fellows in one unit, while the rest carry the usual short spears, and a few light javelin types. Remember, as a whole force, only one man in 10 was issued a firearm, and then allowed only five rounds. I wouldn't say it universally, but if carrying a rifle of some description (at least some Martini's were provided), I'd say no shield or other close combat weapon would be carried by such rankers.
The Natal Native Contingent should look pretty "prickly" and not particularly soldierly--through no fault of their own.
Great troops for war games deserving to be represented in some serious numbers in any collection.
TVAG