SPOTMYUV™ needs light to work. When it's exposed to sunlight (UVA rays, UVB rays, & visible light), SPOT™ changes from clear to purple. When exposed only to visible light (UVA & UVB rays are blocked by sunscreen, for example), SPOT™ changes from purple to clear.
Since sunscreen blocks UV rays, but lets visible light pass through, SPOT™ is clear when your sunscreen is protecting you. As your sunscreen wears off and is no longer protecting you (allowing UV rays through to your skin), SPOT™ changes to purple. We know this can be confusing, so maybe a picture would help:
With our patented skin-mimicking technology, SPOT™ loses sunscreen at the same rate as your skin does. Once you have an SPF of 10 or less on your skin, that's when SPOT™ turns purple to give you a preemptive warning that you need to apply more sunscreen.