Color Splash Photo is a great app that allows you to easily manipulate photos, making particular elements stand out in color whilst the rest of the photo is presented in black and white (think the girl in the red dress from Schindler's List / at least one shot in every wedding album).
The free version of this app gives you a degree of control, but you are limited in the size of the pain brush you can use (and hence the level of "detail" you can control). The paid version unlocks plenty of freedom, but give the free version a go first to make sure it is your sort of app.
For anyone who likes experimenting with photos, this app is definitely worth a try!
In the words of the developer:
Create dramatic images by converting images to gray color or apply colors to your images. Gray out everyone in an image, except for you.its a free application.
Change the color of anything!