The sight of George coming home to rain, no wife, and no dinner; his routine shattered, he wanders to the bowling alley in a stupor to seek guidance from his friends who are thankfully constant.
In the climactic scene, David getting George to finally understand why he misses Betty; not because of the housewife routine, but for the fact he loves her, which George realizes and understands for the first time. And the realization, along with his tears of sadness, "colorizes" him.
The scene where Betty gains color and looks absolutely terrified.
"I don't know what I'd do if I couldn't paint anymore, Bud." If you have something you love doing as much as Bill loved painting, this is gutwrenching.
Betty's goodbye to David. She knows full-well by this point that he's not her real kid, but she doesn't care; she just hands him a brown bag of pastries she'd baked up for the road like any good mom would. And when David returns to his own reality, the bag returns with him.