This is possible in the iOS app that I built [1], which accesses your Kindle library through the Kindle Cloud Reader web app and injects a javascript that allows advanced coloring options. This is used primarily to enable our line-wrapping color gradients, which facilitate visual tracking, but you can also add colors without the gradients. Here are some screenshots of green/black [2] and amber(ish)/black [3]. The app is free to download and this feature has a free trial so you can see how it works. You can also use our Chrome/Firefox/Brave extension [4] for reading on the computer.

Unfortunately we cannot build a similar tool for Android, as Amazon blocks the Kindle Cloud Reader on all Android devices (including the Kindle Fire).

1: https://itunes.apple.com/us/app/beeline-reader/id938026867?m...

2: https://imgur.com/a/nFzLNIs

3: https://imgur.com/a/52kEvi5

4: https://www.beelinereader.com