colouring drawing parts

Does another way exist to fill spaces with colour than by using the hatch key?

What is wrong with a solid hatch? What are you thinking of?

The solid hatch seems to require a closed line around. As an example, the square between 2 sets of 2 crossing parallel ongoing lines cannot be filled. A screen of such lines (as in a fence) cannot be filled at all.

True what you say. I know that in more advanced commercial CAD packages there are more convenient tools for this. Not so in LibreCAD.
You have to redraw the shape on an extra layer, no problem with the proper snaps enabled, and do the hatching there, or on another extra layer. Of course, if you have many items, complicated and tedious.

To color individual parts of a drawing in LibreCAD, you can use layers and assign different colors to each layer. For this:
1. Open a drawing.
2. Layers tab
3. Press the "+" button to create a new layer and name it "Part1" for example.
4. Select the object belonging to "Detail 1".
5. Right click on the selected object, select Edit Layer and select Part 1 as the target layer. This will assign the selected objects to this layer.
6. Repeat steps 3 to 5 for each object you want to color differently.
7. After assigning an object with different words, you can change the color of each layer. Right-click on the layer name in the panel, select Properties, and in the dialog box, you can select the desired color for this layer.

By organizing your drawing into separate layers and assigning colors to each layer, you can achieve the desired coloration for different parts of the drawing.

So I found professional information, that is, LibreCAD users Manual, which may come in handy.