Dirt
The ring around the bathtub is caused by soap scum an dirt. The ring can be washed off using bathroom cleaner.
Bubble bath residue and epithelial tissue.
soap scum sticks to the side of the bathtub witch causes bathtub rings to form.
Your bathtub. Go clean it.
Boron is a metalloid that can prevent bathtub ring by acting as a water-softening agent. When boron is present in water, it can help reduce mineral deposits that cause bathtub rings from forming.
stratum corneum
The color of the ring that the cat leaves in the bathtub in "The Cat in the Hat Comes Back" is pink.
PINK
This might refer to the dirty ring left around a bathtub when someone soaks in a bathtub with soapy bubbles then empties the water. The dirty soap clings to the the sides of the bathtub leaving a ring mark of residue where the high water level was.
PINK
PINK