Certain lighting conditions will desaturate the color of given scenes. This often happens in pictures taken on cloudy days. Sometimes the subject itself may not be vert colorful to begin with. Below is an example of how a photo of the San Francisco Bay Bridge was enhanced to give a little more punch.
Here are the steps:
1) I chose a natural looking orange, representing sunlight, to cover the sky area with. This was chosen from the color swatch palette. In Photoshop, choose Window > Swatches. Drag your mouse over the color you wish to choose, then click. The selected color will show up at the bottom of the Tools window.
2) Near the bottom of the Tools window, click on the curved arrow by the color, so as to bring the selection of the background color forward.
3) Again, use your mouse to select another color from the swatches, perhaps an opposing one. I chose a bluish color to closely representing a natural sky. This second color should now also appear at the bottom of the Tool window.
4) Create a new layer in Photoshop above the original image. In the Photoshop menu, go to Layers > New. From the layers tab in the layers window, select the "Overlay" blend mode.
5) From the Tool menu, select the blend tool and be sure that the leftmost linear gradient of blend in selected above for that tool.
6) Now, drag your mouse from the top center of the image down and let go. You should see a gradient blend between the two colors. Try to get the middle of the blend to be positioned in a natural transformation point, like a horizon, for example. Drag your mouse again to try to get it right. There should be no need to erase or replace the layer. A longer drag will give you a smoother transition.
7) Adjust the opacity of the layer to give you a natural look. Too much opacity will look unreal. Again, be sure the layer containing the blend is in the "Overlay" mode to start with.
8) Try different blending angles, colors and different layer blend modes to see what works best for your image. Click the blend layer on and off to see the effect difference.
9) The last step is to flatten and save the image for sharing.