The discoloration and different texture in the center is likely from froth created when stirring in the evaporated milk. You pie is still going to be edible and delicious! You can cover it up with whipped cream if you need to. To avoid it next time, very slowly stir the milk in. Also after shaking the can of evaporated milk, pour into a bowl and allow it to sit for a few so the bubbles start to disappear before pouring it into the pie filling.