I'm an avid user of the Thunderbird desktop email client and even though development has been put on a backburner by Mozilla, it is offering all the features that I require.
What I like about it especially is the fact that I'm in full control of all emails. If you use an online service such as Gmail or Outlook, you are at the mercy of the provider in regards to email storage and availability.
If the service goes down temporarily or permanently, you may lose access to emails in the process.
Back when I started using Thunderbird I created a sophisticated folder structure and combined it with the program's filtering options to automatically sort emails into designated folders.
While that works fine up to this day, I always disliked the fact that there was no option to distinguish folders visually from each other.
That would be fine if all folders would have the same priority, but since this is not the case, it may make sense to highlight some of the important folders or at the very least change less important folders visually so that they blend in more with the browser.
Color Folders adds one feature to Thunderbird: the ability to change the default yellow color of custom folders to another color.
To modify the color of a folder right-click it, select Color Folders from the context menu, and pick one of the colors displayed by the extension.
A click on more colors opens another palette with dozens of additional colors for you to select.
The colors are only shown in the left sidebar, and only for the selected folder but not subfolders of it. If you want to paint a folder structure in a specific color, you need to do it manually for each folder.
My system, in case you are interested, is to use colors sparingly only to highlight important folders. I use the same color for folders of a certain topic, e.g. business or private folders, which I think is better than using different colors for these kind of folders.
Color Folders makes sense if you use custom folders in Thunderbird. Since it cannot modify default folders such as inbox or spam, it is only useful for that. I'd really like to see a version that adds folder icon modifications as well, but there does not seem to be an add-on available that can do that.
Colors are the next best thing and if used sparingly, help highlight folders of importance.