Dye ink pads are made with water-soluble dyes, which means they'll eventually fade if exposed to sunlight or other harsh conditions. Pigment ink pads, on the other hand, are made with pigment particles that are suspended in a liquid. These pigment particles are much larger than the dye molecules found in dye inks, so they're much more resistant to fading. That's why pigment ink pads are often used for stamping projects that will be displayed outdoors or in other high-light conditions.