While I was poking around the depot I spotted a bright red caboose that no photographer could resist. Unfortunately, while the train car itself was very pretty (see photo below), it was sitting in a somewhat mundane setting with a lot of clutter around it and a blah white sky above it. No shot there.
But one thing that really caught my eye, however, was a series of bright yellow handrails (I think they were hand rails, anyway) contrasting against the shiny red paint of the caboose. Color contrasts make great design subjects because they startle the eye--causing our brains to ping back and forth from one color to the other. Often color contrasts work best when you have a hot color (a red rose) against a cool background (green foliage), but as you can see, even colors that are close to each other on the spectrum, such as yellow and red, can be equally exciting.
All that you really have to do to make color contrasts work is to isolate them--crop away (in the viewfinder) anything that detracts from the intensity of the contrast. In this case I was lucky, the yellow handrails were more or less surrounded by bright red paint, but I still cropped the shot in such a way that the yellow line ran the length of the frame, intensifying the contrast. And again, I was lucky that the line had such a nice natural curl and that there was a strong vertical line of rivets alongside it.