Stir together 1/2 cup warm water and 1 tsp. vinegar.
Combine flour and salt in a large bowl. Slowly stir in water mixture with a wooden spoon until a stiff dough forms, adding up to 1/4 cup more water, if necessary. Knead until smooth and workable. Divide dough into portions, and add desired amount of food coloring paste. Shape dough portions into blocks. Store in refrigerator in a zip-top plastic bag.