The "yellow crayon moment" apparently refers to season 6 finale in BtVS where Xander stopped evil!Willow starting Apocalypse by reminding her he loves her. And that love started since they were children. Wee!Xander saw wee!Willow broke a yellow crayon back in kindergarten and it stuck in his memory. Anyway musesfool has a good & detailed description of the scene here (http://musesfool.livejournal.com/1705774.html#cutid1).