Soak brown and yellow mustard seeds with dark beer in a large bowl; set in the refrigerator for 24 hours. If the seeds soak up the beer too quickly, add more beer.

Transfer soaked mustard seeds to a food processor along with garlic, apple cider vinegar, brown sugar, salt, and black pepper. Pulse until desired consistency is reached.

Sterilize three 1/2-pint canning jars and lids in boiling water for at least 5 minutes.

Pack mustard into the hot, sterilized jars, filling the jars to within 1/4 inch of the top.

Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles.

Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids, and screw on the rings.

Refrigerate the jars of mustard for 2 weeks before using.