They often interact as teacher and student. Luna, with her vast knowledge of the old magic, can teach Twilight things that are not found in modern magic books. For example, Luna might show Twilight how to communicate with the spirits of the night.
Well, the story behind the 'Twilight Music' in 'Twilight Princess' could be tied to the game's plot and the world of twilight. Maybe it plays a role in revealing secrets or triggering certain events.
In general, the Twilight Princess manga is regarded as canon. It adheres closely to the key elements and plotlines set in the original franchise. Minor deviations might exist for artistic or narrative purposes, but the core remains consistent.
In Twilight fanfiction, the 'color of loneliness' could be depicted in various ways. For example, it might be the grayness that surrounds a character when they are separated from their loved one. The vampires in Twilight often experience a sense of isolation, and fanfiction can expand on this. It could be shown through their solitary hunts in the forest, away from the company of others, with only the cold moonlight as their companion.
Their relationship could develop through shared goals. For example, if there is a magical imbalance in Equestria that only their combined magic can fix. As they work towards this goal, they start to understand each other better.