[BLUEPRINT]: Make the dependency graph colour scheme explicit

[pitmonticone]

This PR makes the dependency graph’s color scheme explicit and easily editable.

Closes #69

[kbuzzard]

Thanks, and also thanks for your comments on the colour scheme. Right now "making the blueprint graph look nice" is low priority, I have realised that it's more worthwhile to focus on sorrys and use the blueprint to give more details for the harder ones. But it will no doubt become higher priority later on (in fact this will help, because I realised when I opened the issue that I didn't really understand things properly, and your comments on it mean that I now have a good reference).