A bit late, but I couldnt find anything else online so I figured I’d post my solution here for anybody else in my position, and in case anybody has a way to improve this.
I’m not good at math so this probably isn’t the best way to do this. It also gets a bit janky with small outline widths. But it works.
Since I’m using a static mesh for a grid I used the texture coordinate and get the distance from each UV point to the edge
then just use a step and lerp to determine which color is picked. Then plug into the base color.
I made the outline quite small but its adjustable.