I don’t know if you use Snapseed or not but it’s become very much a part of my iPad/iPhone photography workflow.
I initially started using it because it has a Frames tool that lets me simply add a border to an image before uploading to Instagram/Facebook. But I’ve become increasingly addicted to a few of the other editing tools as well (tonal contrast, glamour glow, define (structure/sharpen), and faces. It also has a very nice healing brush as well as a dodge/burn tool that I use in monochromes.
This morning Google updated Snapseed to add a new Curves tool – which does what you think it should do – including allowing adjusting curves by red/green/blue channel. Very nice update/upgrade – this tool had become extremely useful for a mobile only workflow and it’s amazing that it’s all free from Google! If you haven’t played with it in a while, give it a try.