Simple, but can get better

The plugin is very simple and do it’s job. But it can get improved on some ways.

For instance, maybe you can instead of creating an empty settings page, use the Theme customizer.

This way, will be a lot easier, less code for you and also, the setting will be set by theme (what is good because each theme has it own colors).

Also, you can set default color values getting this info from another settings (like theme background or theme header background).