Your Cadillac probably has the red Dex-Cool in it. Top it off with a 50/50 mix of red Dex-Cool and distilled water or buy some that is premixed 50/50. Green can cause issues such as cooling system deposits and break down of the antifreeze.
Do I have to drain completely and then put in fresh red mixed 50/50 with distilled water
That is up for debate. It may depend on how much green stuff you put in. The Dex-Cool is good for 5 years so if the coolant hasn't been changed in a few years I would do a coolant flush and replace the antifreeze.
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