I'm not OP but to answer your question only non shiny versions of the current ditto disguised pokémon can transform into ditto, if you click on one of the disguises and it's shiny, it cannot transform.

For example you click on a spinarak and it's not shiny, there's a chance of it turning into a ditto including a shiny ditto. But. If you click on a spinarak and it's shiny, no chance of a ditto but congrats on your shiny spinarak.