Certain objects like parts, racks, modules, etc. can get a custom color.
This is done by drag-dropping colors from a color picker on such object.
Each project has a color picker with its own preset color palette.
You can open the project's color picker via the main project menu, or via a shortcut to that function. The default shortcut key is [C].
You can also drop a part or rack on one of the color picker color fields and then drop it from there to another part, that way you can copy colors from one part to another.
Within the color picker itself you can also drag-drop colors so to copy colors or rearrange the color palette.
Right-clicking a color box gives you extra options like Copy, Paste, Edit RGB, etc...
Via the options button at the top-right of the preset color palette, color palettes can be reset to the default color palette, and can be opened and saved into XML files.
You can also drop such XML file on a specific palette component, that will load the XML from that color component on. For example: If you drop a XML with 32 colors on palette color 16, then the first 16 colors in the XML will be loaded into that palette colors 16 to 32.
Feel free to edit those XML files. For example you could create some sub-palettes with only eg. 8 colors and drop them as you prefer.
Sometimes there also is a special color with an asterisk icon in it.
This is used to "uncolor" an object so it uses the default color again.