I'm a big fan of hitting the patchwork nature of the Ynnari pretty hard. I'm working on showcasing that my Ynnari are a mix of different craftworlds, shrines, and kabals and cults.
The Ynnari color (or more accurately, Yvraine's livery color, which has become Ynnead's color) is red. According to the initial release fluff, it's very common for Ynnari to recolor themselves red entirely, or just a little to show their new allegiance. (Which I love, since it gives Eldar something similar, design palette wise, to Deathwatch.)
Long story short, I think it would be cool to pick any existing Craftworld and then just do their color scheme, but with one arm/leg or their face done in red.