Would love to know as well. I'm currently making my own skin and struggling to edit some parts of the UI. I opened some .COL files using notepad, changed some values and it actually changed some things in the game, the problem is that I didn't know what I was doing.
WIsh there was an easy way to find out what the .ICN files represent in the game exactly, so that we don't need to change things through trial and error.
The .ICN and .COL files originate from the original Capitalism 2. The tools used to create these formats are very old and no longer run on the current version of Windows.
The development team has been gradually replacing these legacy assets with new PNG files whenever an interface is updated. For example, in the upcoming post-release beta version, the Manufacturer Guide will receive a UI upgrade, and all new interface elements will be in PNG format. Once that version is released, you’ll be able to modify the Manufacturer Guide’s UI image files.