I met up with a few photographers in town to pick their brains on how to develop the camera filters.
One of them mentioned that the trick Instagram uses is "flattening" of contrast and colors. By evening out the difference between the darkest part and the lightest part of a picture, it allows color filters to be applied evenly without distorting certain parts of the image.
One of them sent me a whole list of filters he uses for his wedding photography. There were some gorgeous filters I could definitely modify and use for my camera app.
I just bought them a couple of beers and dinner, and it was totally worth it! :D