Color Isolation

Step 1: Open your picture in Photoshop and select Layer > New Adjustment Layer > Channel Mixer from the main menu. Check the box marked 'monochrome' and adjust the brightness percentages to suit your personal tastes. Take your time in this stage balancing the picture. There is no ideal way to do this, but since this step dictates the overall look of your final image, experimentation is the key. When you are happy with your results, press OK.
Step 2: Select the new adjustment layer that you have just created from the Layers Palette. Now, select a brush toolwith 100% opacity with a non-fuzzy brush tip and ensure that the colors in the color swatches are Black & white.

Step 3: Using the black color, carefully brush over the parts of the image that you want to be in color. You will need to switch to a smaller brush to isolate the smaller details. If you make a mistake, paint over the area again in white to restore the monochrome effect.

And that's all there is to it!