The XC livery is a vinyl over the original Virgin paint.
As originally planned the XC colours would have been a more grey colour than plum - see the render below from when they were awarded the franchise - with vinyls over the Virgin red ends.
However, somewhere along the line someone messed up and the vinyls were too thin. Therefore there was quite a bit of colour bleed of the original Virgin red through. Consequently XC had to redesign their branding to fit the vinyls that had been ordered to be 'plum' than the originally planned dark grey. This created a new colour which had to be matched for the vinyls to be printed for the 170s and paint for the HSTs.
Original renders:
View attachment 141845