Colortuner

Colortuner by Zefei Xuan is a colour scheme editor that allows you to alter the scheme with sliders. There’s a slider for brightness, contrast, saturation, and hue. You can also invert the scheme to see the dark/light version.

The interface also allows you to switch between themes, and you can specify a list of themes with g:colortuner_preferred_schemes.

The only downside of this plugin is it depends on either a GUI version of Vim, or Neovim in a true colour terminal. Otherwise it looks very cool, and makes experimenting with themes easier than editing files.