Line Work: Colouring in and Hiding Line Layer

After you make the selection you need to expand it slightly. You will find that in the Selection menu. (I can’t remember if it says expand, or grow.) The necessary amount will depend on the size of your image. I’d start with at least 3 pixels. After you expand the selection you can fill the selection with a color on a layer below the line drawing. Set the line art layer blend mode to multiply. Now the color fill tucks under the black lines so you won’t see that white halo anymore.

If this is going to offset printing, follow rob day instructions above. If it is going to be inkjet printed, you can leave it in the RGB colorspace.

As I need a final render which does not have gaps between shapes and no black lines. I have tried matting them.

What color mode are working in and what is the output going to be? If the mode is RGB, the black lines will be converted into 4-color black when the art gets converted to CMYK for output—the gaps won't be black only. For example here I'm hiding the CMYK Black channel after the color conversion:

The accepted method in the comic colouring business is to select an area you want to add colour to by using the Lasso too, and alternate between the normal Lasso and the plygon Lasso by holding down the Option (Mac) or Alt (Windows) key.

Then pick a colour, and fill the selected area with the Option-Delete / Alt-Delete (or Backspace). Do this on a new layer beneath the original artwork, and be sure to create overfills.

The Fill tool is only partially useful for this type of work, because it does not overfill anti-aliased edges, nor is it very controllable when gaps exist.

A faster method (depending on your art) is to use alternative software to create the flatting and holds, and continue to work in Photoshop afterwards.

Tutorial: Coloring with "Colorize-mask"

How to Color Line Art the FAST Way - YouTube

Please post meaningful screenshots including the pertinent Panels.

Expanding the Selection before filling might be an option but I suspect your approach may be flawed.

What are the ultimate output requirements?

Have you considered using Illustrator for this?