Just a guess/speculation (with quite a bit of wishful thinking):
Fairphone will sell the black and green protective case until their stocks are depleted, then they might produce blue and orange ones (runner-ups in the 2020 voting). This would again offer both a solid/conservative/unobtrusive option as well as a vivid eyecatcher. Plus blue is kind of the Fairphone company colour. The idea of a translucent case seems much more unlikely now since the FP3+ came out in an all opaque body (so things have changed significantly after the 2020 poll closed).
I actually believe it is not necessary that Fairphone “always offers colour X”, it is fully ok if they change the available colours every now and then. People can still swap if they are really dying for one that is “out of print”. Of course, there are also reasons why my pure speculation might never materialize. For example, the current colours might just sell so well (I always thought that green was probably desired by many since day 1 of the FP1) that Fairphone just won’t take the risk of discontinuing and replacing them with others.