From Softease Software:
Color Splash Pro is a cool photo app to help you quickly create cool photos by converting them to black and white, while leaving the interesting areas in color. That means users can easily transfer photo between color and black and white, but keeping photo quality and details. That would be the easiest way to change photo style and get a monochrome on photo. Besides turn the whole photo to grayscale, you can define the special items on photo and splash of color on them.