This paper introduces Nail pHolish: a colorimetric biosensor nail polish that changes colors when chemical reactions are detected in hand-fluid interactions. Fingernails are in direct contact with a variety of fluids from both the body and the environment, making them an ideal medium for non-invasive monitoring. Nail polish is used as a substrate for biosensors due to its broad range of colors, durability on fingernails, high visibility, and simple reapplication. This work implements a nail polish formulation and mobile app. The formulation is skin-safe and employs anthocyanins with similar colors to those of traditional nail polish. It is showcased in a series of designs featuring conventional nail art. The app converts color readouts from a portable spectrophotometer to pH levels. Technical evaluations conducted on fake nails show the responsiveness and reversibility of color change at four different pH values. We performed durability tests for over 72 h on 20 fingernails from human subjects. We conducted expert interviews with five nail artists on the practical application of Nail pHolish, and a user study with 10 participants in a dental erosion scenario. This work shows the potential of colorimetric biosensor cosmetics to monitor various bodily fluids such as saliva, sweat, and vaginal secretions, as well as exposures to external fluids such as beverages, rainwater, and swimming environments, offering a new interface for health and environmental monitoring.