While I have no insider insight into this at all, I would really not expect a return of transparent cases anytime soon. Fairphone burned their fingers with those twice. And frankly I do not think that the failure was down to some production mistake here or some mistake there … I think it is no co-incidence that you never (or almost never) see transparent bumper cases on smartphones in general. Most likely there is an inherent problem with achieving both, a transparent colour and a durable and sturdy smartphone case. People sometimes bring up silicone as an option, but I think its elastic quality would not be sturdy enough to keep the FP2 battery in place (plus I find it difficult to imagine it would allow to “click in” the rim – not sure if silicone allows for such tiny details).
Regarding black, it emerged two years ago that a black case was prototyped, but “didn’t look quite good”. No more explanation. My speculation is that a black case would basically equip the FP2 with 3-4 different shades of black on the outside: Case back, case rim, buttons and shiny black areas at the top and the bottom of the display. I think that rarely works: The same colour with several different kinds of surfaces – looks neither fish nor flesh. It’s neither contrast nor harmony.
As I said: I have mainly speculated here.