Experiment with the text colours for Terminal Quest

[CarolineClark]

We need to be consistent in our text colours. Currently we have assigned:

• Red for the hint text.
• Yellow for anything that the user should type exactly, and for showing the echo options for the user.
• Lilac for general hints. It is used for suggesting the right command word but not the whole command, or to highlight the user action.
• Green to congratulate the user.
• Light blue for user speech.

There is two uses for yellow, so it would be good to add a new colour. These are the places the colour is decided https://github.com/KanoComputing/terminal-quest/blob/master/linux_story/gtk3/Storybook.py#L183-L193 https://github.com/KanoComputing/terminal-quest/blob/master/linux_story/gtk3/Storybook.py#L299-L317

and here is an example of how the colour is added to text. https://github.com/KanoComputing/terminal-quest/blob/master/linux_story/story/challenges/challenge_1.py#L26