as touched on before, if you get 100% anti-freeze, you must add water.
"Anti-freeze" is just that, it keeps the water (which cools down the engine) from freezing. that is why buying a pre-mixed 50/50 blend is best.

Dex cool (orange) has had some issues with plastic gaskets, I dunno, I had my 00 Burban flushed, and went back with Dex cool. My Lumina originally had dex cool, but I (don't mix the two) flushed went back with green (glycol) instead.

I've got a 92 K2500 and it has green in the radiator, do I put straight green or 50/50 green/water?

I'd use premix glycol (green)

Depending on where you live a 50/50 mix might not be strong enough. I live in Nebraska and we do sometimes have 20 degrees below zero here. I sometimes will get a gallon of 50/50 and a gallon of 100%. once it's flushed add 70% of the straight and top off with the 50/50. I looked at this thread because I'm going to flush my new to me 1995 Blazer. What's in it now might be orange because it looks really rusty. A while back Westlake Ace hardware had a special on prestone green 50/50 that was $4/gallon limit 2 so I figured $8 was a great deal.