Hard to find a touchup that matches closely. There appears not to be a paint code. I ordered a touch-up kit from a seller in the UK which was certainly of good quality, but the color was off a bit.
I guess I got lucky then. I just purchases 2 used Factory Rado Grey snowflakes off a 2018 ST from a local seller to replace two on my 2014 ST that we're a bit chipped (and one possibly bent). Fortunately they are the same shade.That said, there's some serious variation in shade even among batches of Rado grey wheels. I had a 2015 ST and now have a 2016 ST... both with Rado grey wheels but the 16s are absolutely darker than the 15s were. There's a lot of variation so any touch up paint is going to be a crap shoot. I've used the volvo anthracite and it's more than adequate.