Toothbrush: The Fading Blue StripWhy colored bristles lose their hue over time.

From the show

Why it looks like that

Distinct patches of blue or green bristles on your toothbrush are not just aesthetic design choices. These tufts function as calibrated wear indicators, utilizing a dye that fades at a specific rate to signal when the brush has lost its cleaning efficacy. This visual cue prevents the use of frayed bristles that can damage enamel.