Kroot take washes very well - Agrax Earthshade is your friend. I went through amazing amounts of the stuff putting together my kroot horde. Obviously if you are on a different colour palette for your kroot (and with their variable biology pretty much anything goes that looks good) then use a different wash.
I think an airbrush would be a really viable way to batch paint kroot, I did not have one when I was putting my massed carnivores together but if I were to tackle more now I would:
Mask / Airbrush contrast colour on the quills/crest
Tidy up details, highlight if still required to make them pop