Because the beef has been "Corned", or cured in a solution of salt-petre dissolved in water, the meat will never turn "grey". The pink color is a result of a chemical reaction between the nitrate (or is it nitrite, I can't remember) and the protein in the meat itself, just like ham is "pink".
Most Corned Beef is made from Brisket, which is a really tough cut to begin with. Honestly, the only way to be certain that your meat is done is to temp it with a probe thermometer. It should read anywhere between 180 - 200 degrees F in the thickest part of the meat.
Take it out, and slice the meat across the grain. It should be OK, as long as it came up to temp. I usually only cook Corned Beef in a CP for 3 - 4 hours.
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