ZiKZ comes with 4 main body addons, Fuel Tank reused from Step Croc, Maintainer and Sideboard Bed reused from TUZes and a custom-made Water Tank, a mix of various addons made at different points of development. While the Tanks do color themselves accordingly to the truck's color
The other two do not, because they weren't made to be colorable
Same also applies to the custom-made ZiKZ's Repair Kit, which I hope is temporal state

It would be appropriate at this point to make all of the TUZes' addons (Tank, Maintainer, Sideboard bed) colorable, like here made by me (having file-wise the same results as you will, model was not modified, just the XML of the model) for example
TUZes' tank already been made colorable exclusively on Tatra 805, so there's no stopping from doing it.
Thanks and have fun.
for real, do the same for Maintainance Frame Add-On