I would paint the Venom to match your Kabal if it is going to primarily transport Incubi. I had this same question myself and ultimately decided that Incubi tend to ride in their employers' rides.
I also painted my Kabal and Cult models in the same scheme, so didn't have this question come up!
Are your Cult and Kabal schemes all that different? Could you possibly use similar colors in a variant scheme?
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