Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
depending on how dark you want it, what about just straight waterbased ink. No DC Base, no activator, etc. IN my experience, even unactivated DC base will lighten a bit.
start with black ink and add your white till you hit your grey.
Not sure what your formula is, but on a Black tee our Olive formula would have no White pigment in it. 2-3% activator, 15-20% pigment load.