Blend-in for a metallic strap

I don't have any experience of painting the watch but if the paint was fine for ABS plastic then I guess it should work... It might rub though.

ordering a custom milled casing with the right colored metal (has anyone a 3d-model of the current case?)

Wow - well, I do have a CAD file, but while you could cut it out, the bluetooth is on a flex PCB on the side of the case, and I wonder whether you'd be able to get any usable signal out of it at all with a metal case :(

I guess one option may be to see if you could CNC cut something that would slip over the outside of the case like a bumper, but it'd going to be a big undertaking!