
I'm used to painting up 40k and fantasy so it's very much a one-uniform-for-all sort of thing. It doesn't really leave too much room for creativity when compared to a skirmish-style gang of misfits. Although, one of my friends did inherit a bunch of dark eldar reavers and ravagers that his friend's girlfriend had painted with a variety of glitter paints...
When the faction models are all together like that I can see what you're talking about Pete and Seb. Especially with the turquoise on the Servants, it's quite subtle but does tie them all together. I appreciate you've both got a lot on your hands at the moment, so thank you for taking time out to post in this topic.
I suppose with a faction like the Guild, with lots of disparate models and then some that almost have their own subfactions (Monkey King's group for example), you could use the triad colour wheel method? You could pick one main colour to tie together the faction as a whole while simultaneously tying together the subfactions with another colour that is part of the triad and just make sure you don't use that colour on other non-subfaction models?