How would I go about changing the color of the sky and moon without a texture pack?

I've noticed that https://www.spigotmc.org/resources/bloodmoon-advanced-custom-mobs-items-life-cycles-events.75095/ and https://www.spigotmc.org/resources/skychanger.37524/ are able to change the color of the sky and moon. How would I do this? I want to be able to change the color of the sky in my own plugin