Combine water, mustard powder, vinegar, flour, 1/4 teaspoon plus 1/8 teaspoon salt, turmeric, garlic powder, and paprika in a small saucepan over medium heat; whisk until smooth. Bring mixture to a boil, reduce heat to medium-low, and simmer, stirring often, until mustard is thickened, 5 to 10 minutes.
Remove saucepan from heat; leave uncovered for 1 minute. Cover saucepan with a lid until cooled to room temperature; refrigerate until chilled.