how to change the colour or opacity of a sphere

You have to do this via material and shader. Take a look at this tutorial.

EDIT: for glass you have to use specific shader - take a look at shaders at Unity Wiki.

The visual appearance of every gameobject is determined by the material(s) assigned to it. Materials define colours, textures, and shaders that determine how the object interacts with light in the scene.

Making a material behave like glass is more complicated than simply making it (semi-)transparent - you need to consider reflection, refraction, fresnel effect, viewer and lighting angle etc. - but learning how to apply a transparent material is a good start, so try reading up on that ;)

Go to any folder and right click. Choose "Create -> Material" and proceed to edit the albedo color to be what you want it to be. If you alter the transparency, which sounds like what you want to do, the key thing is that you have to change the material's render mode to "Transparent"--if it is set to opaque it will ignore your alpha choice on your material. Go to your object's mesh renderer, toggle "Materials," and change Element 0 to be your material by either dropping it in or clicking the little target doodad and picking your material from the list.

I'm not sure why everyone else here is a dick and provided useless links you could find yourself on Google. The point of this forum is to provide answers.