Well, if its for 4.15 and up, I would use a Map-Container (String, Colour) for example.
Then I could use it something like ColorPalette.Get(“DarkBackground”), if you get what I mean?
Otherwise you could have a Blueprint Function Library which returns constant values. Then people could change these constants to have their own customization. This wouldnt allow for runtime customization though.
You could also create a new asset type with your parameters for customization as UPROPERTYs and people could set up different configurations and even change them at runtime.
There are many ways to do it, it depends on your project…