This is an Add-On for thunderbird and / SeaMonkey that adds two dropdown menus to the format toolbar inside the compose window,
to quickly highlight and colorize the current selection.
Standard colors are configured: yellow, cyan, green, pink, red, and also light and dark grey.
The current version is basic, but should be sufficient for the majority of users.
Once you have chosen an highlight colour in the drop down menu, you can just click on the button again to use the last colour.

To remove the highlighting or color of a block, simply click inside the block, and select clear from the corresponding dropdown menu. You can clear multiple blocks at once by first making a selection
that includes all.