Unless Yamato makes a new mold for the hand so that the red collar and the back cover of the hand are in separate pieces, you will have the hand molded either in red or black. Alternatively, you will have to paint it either black or red.
There is a reason that Yamato don't paint the hand considering the amount of traffic and friction that you will get when you rotate the hands in and out during transformation.
I'm glad that they didn't paint the hand, or you will be crying paint loss.