We can use colour creatively to change the mood of an image or create a topical interest. The lamppost in this photograph is from a bygone era, where ornate metalwork was de rigeur, and is rarely used in modern manufacturing or construction. The sky was shot early dusk on a winter's day, and while it helps to pronounce the form of the lamppost, it adds minimal value other than providing a good contrasting backdrop.
That said, we can use unusual colours to make the image a little bit different. Of course any colour could be used but the green adds intrigue to this image. Green skies, like the lamppost are quite unusual and the two work well together.
It also reminds us, as photographers that we need to look around (and up) once in a while to appreciate things that on most days might be missed.
Settings used in this particular example are a SOLID gel, in a medium green using the OVER blend mode. The contrast was set to HIGH to boost the difference between light and dark, and while the colour is strong the colour from the original image itself was retained using COLOUR image mode.
Try it for yourself with a variety of blend modes and colours for the right effect.
You can download Geló for iPhone and iPod here... itunes.apple.com/app/id584055229?ls=1&mt=8