Imagine there are 4 objects namely A,B,C and D. Now I want to highlight object A in my animation. If I do so, the Object A alone must glow or must be accompanied by some colour change or only the object A must be visible completely and the objects surrounding it may be applied with vignette effect and must be partially visible.
Could anyone provide your valuable suggestions for achieving this animation effect?
I usually finish rendering and use editing programs or using Composing.
However, if you want to apply it directly to rendering, wouldn’t it be okay to make an image to use for the vignette effect, place it in front of the camera, and give it an animation.
You can think of 2D editing in 3D space. 
I would use compositing. Isolate the characters, and apply and post-process you want on them.
To isolate the characters, I would use Cryptomatte, either object or asset (both to be safe). First enable it in the View Layer’s passes:
In the Compositor, plug a Cryptomatte node, set the layer to viewlayer_name.CryptoAsset. Duplicate it, and set the layer to viewlayer_name.CryptoObject. Plug the Pick output from the Asset cryptomatte to the viewer node to see the different elements it allows you to select:
The Asset cryptomatte tried to differentiate each “asset” by hierarchy. Depending on how your characters are made, it might allow you to select a character in one or a few selections. And for any cases where the asset cryptomatte layer doesn’t allow you to select what you want, use the object cryptomatte instead.
To select a color, click the + icon, it will turn your cursor into an eye dropper, and you can click the color you want. You might need to select multiple areas. You can use the image output to see exactly how it looks, you can also use the matte output, which acts kind of like an alpha channel which is easy to use with other nodes as a filter.
On a practical note, I find it nice to make one Cryptomatte node per selection. So here in my example scene, I make one for Snow, and one for Rain:
From there, you can apply any effect on the isolated character and animate the effects too. And use Alpha Over nodes to combine the final results.
For example if I want to add a yellow glow effect on Snow:
Most of the nodes have a size or factor which can be animated to turn them on/off through time. You can also add a Mix RGB node after a group of nodes, mix it with the originial crypto character, and use that node’s Factor as a way to animate the whole group of node’s presence through time.
