Some of the colorants we use are more potent than others. If any oils or residue are left in your shower or tub, these colors may stick to those oils and give the appearance of a stain. Rest assured, this will wipe out with a magic eraser or cleaning solvent; just clean as you would clean any soap scum. We are not responsible for any stains in tubs due to already existing cracks, scratches, unfinished areas, etc. that were present before using our products. As long as your tub and/or shower is sealed as it would be in a new condition, our products should not cause any problems or stains.