This is mostly for @Numerist but others might be interested. The reason for 64 colours rather than something smaller, like 16, is due to the way colours work. Displays produce colours using red/green/blue sub-pixels. For the Bangle2, these three sub-colours are 1-bit meaning the total colours produced are 2^(3*1)=8. For the Pebble Time the sub-colours are 2-bit (just 1 more bit each) and the total colours are 2^(3*2)=64.
I suppose a display manufacturer could take inspiration from digital cameras and come up with something based on a Bayer filter - 1 bit for red, 1 bit for blue and 2 bits for green: 4 bits total per pixel or 2^4=16. I've never heard of anything like that, though, and suspect the Pebble Time screen is the simplest next step up from what the Bangle 2 has.
Oh, and -several million for a round display. If you want a smart watch over a regular watch, you're wanting to display information - text. There's a very good reason your phone, PC, and even books all have rectangular displays and not circular ones.