Add color support

[klimeryk]

Pulling out of #123 (comment).
Since ReMarkable 1/2 should automatically convert from color to grayscale, might be worth using colors by default that work well with RM1/2. That would keep the existing devices support, while also making the experience on color devices (like the ReMarkable Paper Pro or even just iPads) taking advantage of their color screens.
Of course, I'm sure someone will gonna want a monochrome version on color devices next... ;)