This tutorial will show you how to turn this ::



Into this ::



Ok. So 1st we are gonna set the picture to Grayscale. So go to Adjust >> Hue & Saturation >> Colorize. Make sure the saturation is set to 0.
Thats gonna give you this ::



Now we need to select the part you wanna change the color of. In this case it's Miley's skirt.
So pull out your Freehand Selection too!



Make sure your settings are the same as these ::



Now carefully select the skirt going from point to point. If you go slightly wrong you will have to start the selection again. (not good if it's a large selection!)
When you have selected the skirt the selection should look like this ::



Now we just wanna smooth it out so we dont have any rough edges.
So go to Selections >> Modify >> Smooth and have these settings ::



So now we want to change the color. You can do that by going back to Adjust >> Hue & Saturation >> Colorize and playing with the settings until you get the color you want. I chose this shade of green ::



Now you're done! But to make the most of your effect i would try enhancing the colors by duplicating the base and setting the top to hard light. That will then give you this ::



Cool, huh?