darken unselected and the whole screen....How??

Unity Engine
1

I wanna highlight selected objs,the way i think is to make all the unselected and the whole screen darken a little bit…How can i do it??

2

could you place a semi transparent black plane in front of all objects on main layer, darkening them, then place selected object either in front of plane of on separate layer rendered by a separate camera “highlighting it”?

3

Or maybe could use lights + layers,
Selected object is moved to a layer, which receives bright light,
other objects are on a different layer and use different dimmer light…

Or shaders,
Selected object uses self illuminated shader, others are affected by light or ambient light…?

Or if you use pro, you could probably do post prosessing tricks…?