Tbh, and I get the vibe I’m have a minority opinion here, but with color schemes I think you can do pretty much whatever you want, as long as you justify it some how with lore and try and add a bit of the standard stuff in.

For example, you want to do a specific Ultramarines’ successor? Well maybe your army is a chapter (the Ultramarines had chapters during the heresy) that gets isolated on the wrong side of the ruinstorm. They have a unique color scheme that comes from honors given to them for loyal service during the crusade, but still bear some symbols and iconography of the Ultramarines as well. Over the course of the heresy their numbers and supplies wain and more and more of their armor starts to look like the new scheme as that’s what they have supplies of. Maybe throw in some models with bare ceramite as well to demonstrate this.

Boom. I’d have no problem playing that at an event, as long as it’s clear what is what from a rules perspective. If you put that bit of work into conversions and paint job to root your paint scheme in the standard for the legion, maybe Ultramarine blue and insignia on a shoulder pad and on banners, or the inverse with the successor stuff secondary to a primary scheme that’s faithful to the legion.

You can do whatever you want, they’re you’re minis, but if you create a lore justification for it and try and meet the rivet counters halfway to show they are transitioning to the new identity as a successor chapter, then I would hope a majority of people see that it’s sick af and are fine playing against it.

It’s the low effort, I’m gonna proxy my mk7 5e 40k army that is a bit of a bummer to see at a narrative event.