How does the Nimitta occur? How does the Nimitta occur? I mean does it come all of a sudden (like torch turned on) as a shining bright white light or does it come as light gradually increasing brightness of varied colors?
I am able to see a faint glimmer of blue circular object but not getting anywhere with it. The brightness and color remains constant for few minutes and it vanishes afterwards. Is it Nimitta? What should I do to brighten it
MN128 has a long long discussion about what to do when nimitta vanishes:

MN128:15.3: “Well, sir, while meditating diligent, keen, and resolute, we perceive both light and vision of forms. But before long the light and the vision of forms vanish. We haven’t worked out the reason for that.”

It's rather lengthy, so I won't repeat it but it starts here:

MN128:16.1: “Well, you should work out the reason for that. Before my awakening—when I was still unawakened but intent on awakening—I too perceived both light and vision of forms.  But before long my light and vision of forms vanished. It occurred to me: ‘What’s the cause, what’s the reason why my light and vision of forms vanish?’  It occurred to me: ‘Doubt arose in me, and because of that my immersion fell away.

And continues for a very long explanation...
Regarding what is actually seen, that too is mentioned in DN16. Specifically, visions can be internal or external; limited or limitless; blue, red, yellow or white.

DN16:3.26.1: Perceiving form internally, someone sees visions externally, limitless, both pretty and ugly. (...)

Notably, one should also not be attached to such visions. Let them come and go. Watch without grasping.

MN111:4.5: And he meditated without attraction or repulsion for those phenomena; independent, untied, liberated, detached, his mind free of limits.

In particular, limited visions will have forms, since forms are limited. Just watch.