I have a 1990 EA GHIA,I have been loosing a lot of coolant from holding container when driving.no external leak's when cold or idling.The dip stick is black when engine is cold.No oil traces in coolant tank.Oil cap is milky white,doe's this mean I need a new head gasket or something else could be wrong? Thank's.

yep.. by the sound of it,its the head gasket is blown...it will get worse, its just a matter of time..??