depends for me..
somtimes i use water
and sometimes i use a witch hazel glycerin mixture
it keeps everything in a good suspension and pretty consistant.
if i need a real smooth non agitated look (black and wash) i just dip right on into my rinse cup (water) strait from my black (what i use to line with)
when you buy greywash it usually is dilluted with crazy amounts of alchohol or witch hazel of some sorts (astringents)..which explains the very agitated (just finished red) look to it ...some people like that look, but its not my style.
sorry if im confusing people..just some of my theory. trial and error.