Sorry if this is an old thread that has been long put to sleep, but I thought those of you who began this topic might be interested.
I too had been looking for this effect for my new mobile phone since upgrading from the S3 to the 2017 A5 and was disappointed to not be able to find it anywhere on any of the app stores, but then suddenly by accident I stumbled across it when I was looking for an improvement on my phones stock camera modes (mainly to find a better camera focussing app, but that's another story that only A5 owners will understand) and alas it does still exist and In an improved form in my humble opinion!
I found an app called "mx camera" and besides it being a great app, in its "fx" section it has a filter called "colour splash" that is exactly what we where all looking for, and it can be used both as an after effect or a live effect so that you can see what you are doing before taking the photo. It utilises a slider bar to change between green, orange, blue, purple and yellow point and works really well to create the look of the grey scale/black and white picture with only the chosen colour showing.
I hope this helps people and that you enjoy playing around with this filter!!