Obviously either Renji hasn't seen Ichigo's Getsuga Tenshou in bankai/bankai+hollow mode before, or the writers are trying to make an emphasis on the power of the attack, by denoting the change in color outright. I did not pay much attention to the beginning this arc, and maybe missed an ep. where he did reveal Bankai Getsuga to Renji, but I think the first option is more likely, because the only other time as far as I can recall that Hollow Ichigo used that attack was against Byakuya waaay back in the last arc.