Sorry I don't know how to translate to Chinese. Basically it is comparing people to different kinds of paint finishes. "Flat" is kind of boring, nothing special. "Satin" is a smooth finish, "gloss" is shiny. "Iridescent" sparkles. So, I guess the meaning is that most people are ordinary, but every now and then you come across a person who is really special.
Hope you can understand my explanation. By the way, I am curious as to where you heard this phrase.