Particle and Fog Background?

Hello, I am currently trying to make a background like this one :


fog%20prob
I would like to make it pink-blue, so I added a blue smoke and pink particles. This is my last render, quite long if you’d ask me (Note : there are more particles in the side, I made a (not so) big background because I will move the camera to explore all the places. I don’t know if there are ways to speed it up)

Do you know how I could add a blue background, to make it look like in the example picture ? I tried adding smoke, but after many (MANY) misfortunes I understood it was not the way to go.
Maybe something more aurora borealis-looking, if you seem what I mean ? I wouldn’t want a block of homogeneous light, but more a background with some irregularities, but which wouldn’t look like it’s just a screen behind the scene.
Also I would need it to be already present at frame 1. One of the problems I had with smoke is that it always started at frame 1 and I couldn’t find a way to edit the start frame.

Thank you in advance for your answers !