Color Splash is an app for the iPad which creates awesome effects without putting much effort. You just have to upload pics on the iPad using iTunes and launch Color Splash. The photo is converted to black and white and appears on the screen with the color tab selected at the top of the screen and you have to add color back into the picture by painting with your finger.
The app allows to adjust the brush size and intensity, undoing accidental brush strokes and pinching to zoom in and out in a photo to ease coloring. It comes with a nice tutorial and easy to use too. The app costs 1.99 dollars. [link]