Add the 4 cups heavy cream, 4 cups whole milk, 16 ounces white chocolate chips, 14 ounces sweetened condensed milk, and 1 Tablespoon vanilla extract to a 6-quart slow cooker.

Stir gently to combine all ingredients.

Cover the slow cooker and cook on a low heat setting for 2-3 hours.

Stir the mixture thoroughly approximately every 20 minutes until the white chocolate chips are completely melted and all ingredients are fully combined and smooth.

Add green food coloring and yellow food coloring, a few drops at a time, until you achieve your desired 'Grinchy' shade of green.

Serve the hot chocolate immediately, or keep it on the warm setting in the slow cooker for up to 3 hours. If the hot chocolate becomes too thick while resting, whisk in a splash more whole milk to reach your preferred consistency.