ORIGINAL: geofflambert
I'm not sure about this, but I think those hexside colors reference land movement. Caligula could order his troops to march into the sea but they might not come back. On the other hand it should certainly be legal to amphibious assault that hex. That would mean ships can enter that hex through that hexside so the question is not what the hexside color is at their entry but rather the hexside color when they leave.
edit: when and where they attempt to leave.
The way I read the hexside colors is:
red: impassable for all
green: only land units can pass
blue: only sea units can pass
white: both land and sea
purple: river crossing will trigger shock attack