Not everything is as simple as adding a gradient, in this image there is a photomontage job, looking in detail each area it's easy to find some repetition patterns
Starting from a background image, you can achieve color effects by superimposing images, adding gaussian blur, experimenting with layers blending modes, opacities, masks and grouped effect layers.
In this quick example:
- Blending modes: Vivid Light and Difference
- Hue and Saturation to change each color layer
Images from unsplash: clouds, colors