Using this vanguard under Mu Yanling, I noticed it was converting gems to black and blue, instead of blue only (-2 ability).
The match was against Nicol Bolas 1, & I tried again playing against Ob Nixilis. It only converted gems to black in that match.
I have yet to test this while playing as another PW, but so far this is pointing to the card converting gems based on your opponent’s color(s).