Photo Delight is a simple to use color splashing app for iPad photography, with a "smart" color feature built in to ease problems with borders and lines.
iPhone and iPad apps tagged "selective color."
Color Range is Next Generation Photo Editing App for iOS
As a photo addict, I'm always on the hunt for iPhone photo apps. The latest and greatest is Color Range, an iOS app that specializes in adding selective color effects to pics in your iPhone or iPad Photo Library.
ColorSplash: Add a Splash of Color to Black and White Photos
ColorSplash, from Hendrik Kueck, is both entertaining and frustrating to use, at least if your fingers are the size of premium hot dogs. It's an image-manipulation app that enables you to convert your color photos into black and white and then restore the color to objects you would like to highlight.