You will also need the app Filterstorm for this tutorial
It takes only a few easy steps to apply selective colouring, regardless which of the different apps one uses for the task. This tutorial shows two methods for applying selective colouring in Filterstorm.
Keep in mind that Filterstorm is a highly versatile image editing app intended for use in a variety of processes. As such, it does have a learning curve. It's also fair to say that the interface is not the most intuitive. (There is a new version coming soon, but I don't know what UI improvements we might expect.)
If you think you'd prefer the simpler interface of a dedicated app, then I might recommend trying ColorSplash. (Tutorial coming soon before end of May 2011.)
Here is my original image.
The first step is to convert the photo to whatever tone you want for the final image. In this case, I've used the Silver Gelatin filter in Camera+.
The second step is restoring the colour (or colours) we want using Filterstorm.
Open the converted image in Filterstorm. Select "Add Exposure" at the bottom of the list of choices under the "Filters" tab. You will be invited to upload an image from your photo library.
Choose the original image as the colour source. Use the brush tool to "paint" in colour from the original. That's basically all there is to it.
There is a shorter route yet, if Filterstorm's "Black & White" filter suffices your needs as the type of monochrome you use for the final image. Here's how it's done.
Open your colour image in Filterstorm. Select "Black & White" from the list of choices under the "Filters" tab. That done, you will be presented with the following dialog.
From here, you can simply choose whether you want to render the image in black & white and then add colour, or to leave the image as is and "paint in" black & white.
One note in closing. Brush size is critical to doing a clean job, and the brush does not resize automatically when you zoom in. Despite the bothersome interface, you will want to adjust it down to a small size when you zoom in and work on getting the details right. Tap on the double-arrow button on the right to pop up the tool dialog and check the brush size relative to the zoom rate at any given time.