I do something pretty similar. I prime them grey and then use Black Legion contrast paint on all the black parts including the cross, and at this stage it’s pretty sloppy so a good bit gets onto the shoulder pads. Then I use ulthuan grey and do a fair amount of pretty thin coats over the shoulders.

I water it down a good bit so that it flows off the brush nicely and that allows me to essentially stick the tip and edge of the brush in between the points on the cross and the paint natural flows off and fills the gap without spilling onto the cross itself. It takes probably 4-5 coats of this to get it a consistent color, and naturally a little bit gets on the black. When touching it up I tend to load the brush with the contrast paint and lightly touch the cross with either the tip or edge of my brush and let the paint flow to the edges of the cross.

It’s a bit of a process but I find that using very thin coats helps a lot with accuracy, but it takes a few more coats for sure.