My suggestion is to find a copy of Ian Knight’s ‘British Forces in Zululand 1879’ Osprey Elite 32. It has some useful drawings of wagons and carts and illustrations of support troops.
All ranks of the Army Service Corps wore a dark blue uniform (p45 of the above). I assume that General Service wagons would be the same grey that limbers and artillery carriages were painted.
There was a shortage of General Service wagons in the campaign, and local ones were hired/commandeered (p24 of the above). The ox cart looks to be one such. The drivers are definitely civilians (Boers?), so the vehicle probably wouldn’t be a military colour IYSWIM. There is a drawing of exactly the same type of ox cart on p28, with the caption “Chelmsford’s existing transport was completely inadequate for his needs, civilian wagons, of which this is a typical pattern, were purchased or hired to make up the shortfall.”
"I'm not signing that"