I just found this description re the Ducky caps:
"...laser foam technology, which means that the black keycaps are added with special ABS materials. When the laser burner touches the keycaps' surface, in addition to the depression created, heated white bubbles fill the entire groove where the depression was made by the laser. This makes the white in the keycap very accentuated. However, by using this kind of keycap, the disadvantage is that playing for a long time may scale the colours due to the user's hand leaving prints."