About

Comparison between the default LUT without any adjustments and a finished modified LUT. Using Relight with default/recommended settings.
LUT stands for look-up table. It's a map of colors which allows applying color correction and adjustments to the image.
Folder
The folder where you should put the LUT png is located here:
%LocalAppData%\Colossal Order\Cities_Skylines\Addons\ColorCorrections
The AppData folder is hidden by default, you can access it by copying and pasting the path into the file explorer, or pressing Windows + R and typing in appdata.
Creation
Default LUT
To create an LUT, you first need to download the default LUT which has no adjustments.
Adjustments
You should only apply per-pixel color adjustments, that means you cannot apply effects such as blur, sharpening, clarity, highlights/shadows, and many other effects which are more than just a simple color adjustment.
You can apply color adjustments such as brightness, contrast, levels, curves, exposure, vibrance, saturation, hue, color balance, black & white, color lookups, gradient maps, and selective color.
16 bits/channel
When many adjustments are applied, working with a 16 bits/channel image can have the benefits of less noise and banding issues. But make sure the final image is exported as 8 bits/channel, otherwise, the game won't recognize it.
Example
Download a Photoshop file of an example LUT with various adjustments. Must be exported as 8 bits/channel png.