On Nemo (version 6.0.2), right clicking on a folder brings a context menu that includes the ability to change the color of the folder. This menu item is right under delete, and looks like a horizontal list of colored squares, the leftmost of which is the "restore color" button. Unfortunately, "restore" the color and "delete" the folder share the same X-looking icon in my system.
I notice this to be a huge issue in user interfaces on Linux. Developers will specify that a button should have a generic icon that is loosely related to their function based on common available icon names, but icon designers only have one concrete vision of what the icon should be look like in their theme, so you end up with multiple buttons with the same icon that perform completely different functions, defeating the point of icons, in my opinion.