Trapcode form how to get colour between lines rather than just the lines?

Do not use strings, use regular planes or boxes with particle densities set accordingly. This will give much finer gradations and a more solid appearance.

Mylenium

Trapcode Form is an incredibly complex 3rd party plugin. I suggest starting with the explanations on the Maxon Form Product Page. Follow up with some of these;

01. Trapcode Form 3 Training - Intro to the Designer

02. Trapcode Form 3 Training - Base Form and Particles

03. Trapcode Form 3 Training - Curves

04. Trapcode Form 3 Training - 3D Objects

05. Trapcode Form 3 Training - Fractal Fields

06. Trapcode Form 3 Training - Layer Maps

07. Trapcode Form 3 Training - Custom Particle Types

08. Trapcode Form 3 Training - Lights, Shadows, and 3D Cameras

09. Trapcode Form 3 Training - DOF, Visibility, World Transform, and Kaleidospace

10. Trapcode Form 3 Training - Audio Reactors

11. Trapcode Form 3 Training - Alien Fire Project

12. Trapcode Form 3 Training - Fantasy User Interface Project

The Audio Reactors (#12) might be one you want to look at first. 

These are some of the best Trapcode tutorials you can find. They are on the Maxon Cineuniversity website. 

You need to show us the details from within the Designer.

Mylenium

As I said, just add a second system which can have completely different properties from your lines.

Mylenium

Filling the gaps would simply mean more particles, so increase the Strings Y or use a second particle grid with.

Mylenium