It's been interesting to see how many new non-Photoshop image applications have been coming out for the Mac. While Photoshop can apply just about any effect to an image, I'm seeing an increasing number of specialized, low cost apps that do one or two things exceedingly well and don't require a mega-investment in Photoshop.
A case in point is Color Splash Studio. The app allows you to take a color image, turn it to grayscale, and then brush selected color back in. This isn't an effect you would use on every image, but it has its place and can look pretty amazing if the subject matter supports it.
Color Splash Studio is easy to use. You start by importing any image, including camera raw photos, and the app displays a grayscale image on your screen. With a brush, you basically paint the color back in, perhaps highlighting a flower or a sunset. If you make a mistake, there is an undo command, or you can brush the grayscale back onto the image if you get a bit sloppy defining an edge.
The app provides zoom to help in maintaining edge accuracy, as well as an inverse mode so you can make an object grayscale while everything else is in vivid color. You can set the opacity and size of the brush in addition to its softness. In the hybrid picture that results, you can adjust the grayscale or color parameters separately including exposure, blur, contrast and hue (if you are working with color). I've enclosed a couple of sample screens in the gallery.
Color Splash Pro is normally US $4.99, but is being introduced at $1.99 in the Mac app store. If you have a need for this effect and don't have a high-end image editor, it's a very worthwhile purchase.