How much DEX did you add to top it off? You mentioned the system was flushed by a shop and just now discovered it was green. Did you contact the shop to confirm what type of “green” antifreeze they added? Napa offers an “Extended Life Cooling System Antifreeze + Coolant for All Makes All Models” and it’s green. When flushing the system I’m assuming you are removing the thermostat first? Just to confirm - the vehicle is a 96 C1500 w/5.7 correct? If possible can you post pics of the radiator fill neck, radiator cap (bottom view), and the thermostat port/jacket? Did you observe gel, chunks, rust, etc. in the old antifreeze?