I learned from this forum that all coolant is not the same ...I thought the only difference was colour. Some come ready mix and some need to be diluted with water. Some people use water only and the car seems to work well except that the radiator need to be topped up more often. I am thinking water may have rust and do cause rust...gets hot pretty quickly, will generate steam , expand and increase pressure in the cooling system... can this result in damage to the engine or affect performance of the car? ... or is it using water only will simply cause you to refill the radiator more often?

Read the car's owner manual for your coolant type/colour. For other answers,
The answer is below

Originalsly: I learned from this forum that all coolant is not the same ...I thought the only difference was colour. Some come altready mix and some need to be diluted with water. Some people use water only and the car seems to work well except that the radiator need to be topped up more often. I am thinking water may have rust and do cause rust...gets hot pretty quickly, will generate steam , expand and increase pressure in the cooling system... can this result in damage to the engine or affect performance of the car? ... or is it using water only will simply cause you to refill the radiator more often?

No car manufacturer would advice any car owner to put only water in their radiator in place coolant. Instead they would always advice you get a special liquid recommended by them and mix part with water (depending on their recommendation) and use as coolant. Water alone in radiator causes rust in the radiator components and also don't have lower freezing point than those liquids neither do they have a higher boiling point than those liquid (recommended liquid+water lowers freezing point and raises boiling point of the radiator fluids).