For some reasons some of Btvs scenes don't appear, but I'll try to work that out soon... =/
The falshbacks aka black and white scenes: Buffy had a long time friend, Lucas, that she left behind when she moved to Sunnydale. A year as passed since she moved, and since they missed each other Lucas moves in with her.
As the time goes by, they understand that their relation is gone far from being just friends, but Buffy is dating Angel, so they hide their true feelings for each other for a while, until Angel's gone to L.A.
Buffy and Lucas tried to make their relation works, but when Lucas understand Buffy will never forget Angel he leaves.
The cloloured scenes is what happens after Lucas returns to Sunnydale