Buy some gentian violet microscopy stain, hard to wash off anything, including the sink, so play with it somewhere marriage-safe! You'll look like one of those purple camo soldiers in the cadbury adverts, but you'll be visible for sure.
This link suggests the gentian violet stain is fixed by heat, e.g. a tumble drier, but I'd hesitate to risk staining the tumble drier. https://breastfeedingpositionsreview...or-purple.html
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