18" is about the height of pixie lilies. Pixie lilies are the names a specific group of hybridizers give to lilies which turn out short. You are correct that they are asiatic lilies.

Pixie lilies are not a specific related strain but a collection of short growing hybrid lilies. The same group of hybridizers who own right to the name, give their short hybrids this "nickname". Therefore, colors range full scale depending on the particular crosses the hybridizers have made. It is not limited in color, shape or otherwise. Only the height criteria is kept.