Most of the coolants are ethylene gycol-based, this is the part that stops it freezing. The difference is in the anti-corrosion technology, so Dex-cool is gycol-based but with an organic acid to stop corrosion (maybe!) and the green is gycol based but with some good old silicates, phosphates, or whatever to prevent corrosion.

Looks like Prestone make the green stuff, it's called Prime now.