OK, I think I finally understand what you meant. You’d have one of those set up for clear skies, and another for rain, and you’d like to lerp from one to the other.
Sorry @Coldscooter I don’t think there’s any way to do that. And I don’t know about using UScript to override Matinee stuff at runtime. If all of those SkyChange tracks control material instance parameters, you can copy all of them and make rainy versions for each: RainyHorizonColor, RainyRimColor, etc. Copy/paste the entire material, and change the parameter names to RainyHorizonColor, RainyRimColor. And then you can lerp between them with a Rain parameter, controlled by the rain matinee.
If that doesn’t work for you, I would next consider loading and unloading a rainy level over the rest of the world. Give it a cloud mesh and a sky sphere that’s smaller than the regular sky sphere so that the rainy meshes block out the sky. Gradually increase the opacity until you can only see the rainy versions. And make a post-process color-grading LUT that desaturates, reduces contrast, shifts towards blue, and darkens.