Hello, I’m pretty new at compositing but i got some ideas in how to use this, but i cant never manage to solve this particular problem:

As you can see the background is black and it supposed to be blue, and if you watch closely, there’s like a deep blue edge that shouldn’t be there.
I only work in cycles and I’ve been trying to simulate mist using the Z property of the render but i almost never manage to make it correctly due to that problem, I mostly always just grab the Z property and mix it with a math converter to multiply it by 0.01 in order to simulate mist. But this technique never works with colored mist… Is there a way to do this much easier?
Thank you.
