Spells to Change the Soul

You may assign an instant or sorcery to be your
Commander's partner (it must have two or
more colours in its casting cost).
It starts in your command zone and its colours
add to your Commander's colour identity. Once
you cast it, it is placed into exile, and can't
return to your Command Zone.

(to use this Blueprint, your original Commander
must have only one colour in its colour identity)