An aquarium official believes the unusual coloring of the marine animal -- Asterina pectinifera known as ''itomaki hitode'' in Japanese -- is caused by its inability to produce pigment.   The starfish was discovered in November by Masahiko Inoue, an official of a local farming fishery association, on the coast of the town of Daisen in Tottori Prefecture at the depth of about two meters as he was releasing abalones into the sea.   ''It was the first time I had seen a white one and I didn't initially realize it was a starfish,'' according to Inoue, who said he has seen thousands of starfish in his lifetime.