Is the purple one made in Hong Kong? I believe there's a made in Italy version with dark purple hearts.
If it's kind of a lavender, powder-blue-ish purple then it's from the paint fading . . . On that color of symbol, sometimes the red pigment will leach out, leaving a purply / blue-ish color behind.
Some of the other ponies it can happen to are Heart Throb, Scoops, Love Melody, TAF Fifi, and Baby Crumpet. I used to have a really pretty Scoops this had happened to, she looked even prettier than the "normal" version.