The purple color means that the color-changing ink has been exposed to UV light.There are many color-changing inks out there, but our R&D team selected this purple ink for a few reasons:1. The color purple had one of the highest contrasts across all skin tones, so it's easy to see the color when you're in UV danger.2. The ink is completely non-toxic and safe, proven by 3rd party experts and labs.3. The ink is only sensitive to UV and visible light, so you can always be sure that the color means you've been exposed to UV. Other color changing inks can change color with heat, which isn't helpful if you're outside on a hot day!