How can I fill this shape?

[nekomuu]

I am trying vector drawing first time.
I used pen tool to draw this black curves and they are in the 1 group.
I used break line button? to go to make next curve line.  (when you tap the right arrow on pen tool, the the icon is the most left.) 
If I tap the group & try to change the color to fill inside, some lines disappears  and leaves the triangle shaped empty space.
Why is this happening? and How can I fix this? 

[MEB]

Hi nekomuu,
Welcome to Affinity Forums
This is happening because you are not using a single closed shape for the whole hair but several independent lines that - as a whole - create a closed shape visually. To completely fill a shape you should ideally work with closed shapes, starting drawing them with the Pen tool and placing the last node over the first one (to close them). If you use just single lines (non-closed shapes) Affinity will simply fill them connecting the first node to the last one directly using a straight line (see image below).

So in your example you would have to draw a shape representing the outline/boundary of the whole hair, then use single lines (over it) to represent the single hair details.

[nekomuu]

Thank you very much for the detailed advice!
I understand what was wrong and I could fix it!

[JustRob]

I used the pen tool to draw four straight lines to create a rectangle, then created a group of the four lines.  I assume it is a closed shape as I can move the rectangle.  I now want to fill the rectangle, and when I select the Fill Tool as a Solid nothing happens.  The Color Studio shows blocked.  How can I fill this shape with a solid color; I do not at this time want a gradient.  Thanks

[DM1]

[JustRob]

DM1, thanks for your help with this.  Apparently I was either making the task too difficult, or going about it incorrectly as the fill did not work on my constructed rectangle.  Thanks again.