This test method is used to determine the resistance to staining by acid food colors on pile floor coverings. 2. Principle 2.1 A specimen of pile floor covering is stained with a small volume of a diluted aqueous solution of Food Drug and Cosmetic (FD&C) Red 40 adjusted to an acid pH. After allowing the stained specimen to remain at controlled conditions for 24 plus or minus 4 h, it is rinsed in water to remove all unused FD&C Red 40 dye. Any residual stain is assessed after drying.