The high level is you have a shader that will change color based on the world space position and radius of a mathematical sphere passed to the shader from a script. You can give it a soft edge by blending in over the distance to the sphere’s surface, and add some noise so that it fades in with some texture to it.
To get to the point where you can do the above you’re going to have to learn a ton about shader programming first though. Look here on the forums for newbie tips and the like, but this is where I tend to send people first: