I have no idea what exact color values used by USPCC, which is OK, since I'm not trying to exactly match their color anyway.
Inkscape doesn't (by default) use pantone values/names, so for the vector backs, I've basically scanned/sampled (or eyeballed) colors from real cards, basically looking for values that look (and likely print) nicely.
I've seen cards printed before (from my vectors) using these values (specifically blue and red) and it looks great. Since the vector backs are not meant to be mixed into any other kind of commercial deck, I'm not trying to matching anything exactly here. The red/blue mix on commercial decks seems to vary, sometimes widely, so it's kind of a fools errand to attempt matching something that not even the big boys can match consistently.
Of course, since the vectors are infinitely editable, changing the values to anything one desires is really easy/quick.