While I still am unsure why the translucence varies, I believe it’s a mixture of how closely they are still tied to the physical earth (more connected to earth, more ‘earthly’ looking, less connected, more ‘spirity’ looking). I also think it has to do with their skill level at being able to appear and the amount of energy they have to appear - it takes a lot of skill and energy to appear as a human in physical form without an actual physical body to work with!
The fraudulent medium Ronald Edwin confessed he had duped his séance sitters and revealed the fraudulent methods he had used in his book Clock Without Hands (1955). The psychical researcher Tony Cornell investigated the mediumship of Alec Harris in 1955. During the séance "spirit" materializations emerged from a cabinet and walked around the room. Cornell wrote that a stomach rumble, nicotine smelling breath and a pulse gave it away that all the spirit figures were in fact Harris and that he had dressed up as each one behind the cabinet.
The exposures of fraudulent activity led to a rapid decline in ectoplasm and materialization séances. Investigator Joe Nickell has written that modern self-proclaimed mediums like John Edward, Sylvia Browne, Rosemary Altea and James Van Praagh are avoiding the Victorian tradition of dark rooms, spirit handwriting and flying tambourines as these methods risk exposure. They instead use "mental mediumship" tactics like cold reading or gleaning information from sitters before hand (hot reading). Group readings also improve hits by making general statements with conviction, which will fit at least one person in the audience. Shows are carefully edited before airing to show only what appears to be hits and removing anything that does not reflect well on the medium.
^ Joseph McCabe. (1920). Spiritualism: A Popular History from 1847. Dodd, Mead and Company. pp. 110–12. A Mr. Merrifield was present at one of the sittings. Home's usual phenomena were messages, the moving of objects (presumably at a distance), and the playing of an accordion which he held with one hand under the shadow of the table. But from an early date in America he had been accustomed occasionally to "materialise" hands (as it was afterwards called). The sitters would, in the darkness, faintly see a ghostly hand and arm, or they might feel the touch of an icy limb. Mr. Merrifield and the other sitters saw a "spirit-hand" stretch across the faintly lit space of the window. But Mr. Merrifield says that Home sat, or crouched, low in a low chair, and that the "spirit-hand" was a false limb on the end of Home's arm. At other times, he says, he saw that Home was using his foot."