Photochromic (color change with UV light).   UV reactive coating makes the color change possible.  When UV light hits the material, either by going outside, near a window or even just shining a UV pen on it, the color will change from the off-white base to a lovely Purple.   When in direct sun, the change happens quickly and color is much stronger!  

** PRO TIP - Try masking off shapes or designs and painting the unmasked parts the same white color as the material base.  It creates a fun hidden image sort of effect when you remove the masking and take it outside!