When drawing on the computer,
or tracing a real pencil sketch...

Use a very thin brush,
as think as you can go.

Because when coloring a circle for example,
you simply use the contiguous selection tool...

With a grow setting,
so that the selection that is created on the inside...

Cuts into your line art,
as a result becomes tucked beneath the line art.

If you have an intricate design,
like eyes on a face.

There is no good reason why there should be a large gap,
when you turn off you line art.

The color of the eyeball and the color of the face,
should almost touch.

You should be able to tun off the lines in your illustration,
and not see holes where the lines were.

If you choose to make your illustration more realistic,
by removing the lines and extending the gradients.

You won't have to go around all the edges,
filling in the holes left by lines that you have now removed.

Using a selection tool to select an area defined by your drawing,
and then filling it with color is not the perfect approach.

But gosh darn it is is fast,
it is almost powerful.

And it is perfect,
if you are just doing illustrations.

Another alternative is,
is not to use the selection tool.

Just use a standard brush,
and paint over your line art like a normal person.

This does not mean you are eliminating your line art,
because once you are done painting...

You can move the line art layer above it.
there are no holes here, no problems.

You end up with an illustration that can have strong outlines,
or no lines at all, just gradients you painted.

But the problem here is that you have to be careful not to go over the line,
which is what the contiguous selection tool can help you with.

The best approach,
is a fusion of both.

Use the contiguous selection tool, along with grow selection under Select menu,
and paint over you line art.

So here the selection tool,
allows you not to worry about going over the line...

And manually growing it will help you stack your layers,
without any holes, and in a way that is compatible with your line art.

So that you can bring it back,
above your painting, if you wish.