Really Useful Colours is a magazine story.
On a foggy morning, James meets BoCo while passing the quarry. He laughs at BoCo's dusty paint, but BoCo asks why anyone would want to be red like James, as red means stop, while green means go. James continues forwards until he reaches a red signal.
Suddenly, several rocks start falling down from the quarry and knock the signal down. BoCo, thinking fast, tells James to stay there, as his shiny red paint will shine through the fog and act as a red signal. He goes off to collect the Fat Controller, and when he returns, the Fat Controller praises both engines, saying that both red and green are really useful colours.