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