I'm not as knowledgeable as to what constitutes a mod, but it does tell the game to use my colours instead of the original, so I'm guessing yes. I had a discussion somewhere on here a while back, and it was decided that when the files are added to the .world file, it's not, but when it's an individual package on it's own, it is. When it's part of the .world file, it doesn't require any additional installation - it gets installed with the world, and only works in that world. But as an individual file, it works in all worlds unless you remove it from the overrides folder.