Mustard as a condiment has a yellow color because of the addition of a plant called turmeric in the production process. Mustard seed by itself has a dull-gray or brown color, whereas turmeric has a yellow color and is used as a natural food dye.
The section of the turmeric plant used to dye mustard yellow is its rootstock, also knows as the rhizome. After being prepared, the rhizome is ground to a powder, which is then mixed with mustard sweet powder, water, vinegar and salt to make yellow mustard. Turmeric was first used as a food ingredient in the 12th and 13th centuries.