Well sorry to be the answer you don't want to hear but...
Yes, Rocky Mt. will process it as BW... which is a true waste of Kodachrome 120 ( oh those great 6x6 slides of yore). Kind of like having a Porche 911 and just driving it up and down your driveway
As for true color slide processing of 120 size Kodachrome... well to paraphrase a statement from a really nice and funny guy...
... "No one in this solar system does it anymore, outside of this solar system, the postage alone would be prohibitive..."
Now I too wish someone here on earth would do this again, do a search here on this forum and check the numerous posts about others trying to do home processing.
Maybe if enough of us shoot what we still have in our freezers, it may just be enough of a push for some one out there to do this ....
Anyway, BW yes, color no.
Not here... , not in Canada, ... not in Europe, ...3rd ring of Jupiter, well the e-mail I just got about a month ago said the guy is on vacation till 2055