In this world of 16.7 million color displays and high megapixel cameras it may seem like a backward step to convert images into black and white but, as modern motion pictures like Schindler's List (and more recently, Sin City) have shown us, effective use of isolated color in monochrome images can be very eye-catching and portray important information & emotions.
As one can imagine, there are dozens of different ways to accomplish color isolation in Photoshop. To keep things simple, however, I will cover the two different methods that I consider to be the best for this task.
Method 1: Channel Mixer Adjustment Layers
This method makes extensive use of adjustment layers to make non-destructive changes to an image, and is most suited to simple shapes and edits. Many thanks go to Bub for the image used below.
 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.
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 Step 2: Select the new adjustment layer that you have just created from the Layers Palette. Now, select a Brush Tool with 100% opacity with a non-fuzzy brush tip and ensure that the colors in the color swatches are black&white.
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 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! |
Method 2: Color Selection
This alternate method makes destructive changes, but also utilizes color selections to make much more complex selections possible. Please note that this technique works best on images where there is a great deal of existing color separation (i.e. in this example, our picture is a red flower set against a greatly green background).
 Step 1: Open your picture in Photoshop and immediately duplicate your layer by selecting Layer > Duplicate Layer from the main menu. Hide the new layer on the top row of the layers palette, and click on the original layer to reselect it.
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 Step 2: Desaturate the layer using Image > Adjustments > Desaturate. Many people do not like the default 'desaturated' look, so play around with the Levels and Brightness/Contrast until this you are happy with the results. |
 Step 3: Unhide the top (color) layer and click on it to make it the new active layer. Now, go to Select > Color Range. This will load up a rarely-used and complex feature of Photoshop. To start, click on a color that you want to select in the main image using the Eyedropper Tool. As you will notice, the Color Selection window will show your selected color as a mask of white against black (white being the selected area). The Fuzziness slider will adjust the tolerance of your selection (i.e. the bigger the number, the more colors NEAR your selected color will also be selected).
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 Step 4: Using a mixture of the fuzziness slider and the Eye Dropper Tools (the + and - tools are essential too, to both add and remove colors from your selection respectively), isolate your image from the background. Don't worry if little areas become selected against your will, because we will correct that imminently.
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 Step 5: When you are happy with your selection, press OK. As you will now notice, you have an active selection around the colors you chose. You should now reverse this selection with Select > Invert from the main menu. Press DELETE on your keyboard, and voila - you have isolated color against a monochrome background! |
 Step 6: Using the Erasser Tool you should now clean up your isolated color layer as much as possible, to remove areas of color that were accidentally selected. And that's all there is to it! You can repeat Steps 3 to 5 in more than one layer if you need to isolate radically different colors - they should stack against the monochrome layer just fine if you keep it simple.
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Alternate Methods
As I mentioned in the introduction there are a great deal of other ways to isolate selections of color. Some people use the Lasso Tool, others use complex masks. They all work, and have their own individual strengths and weaknesses. My advice is to just experiment and see which method works best for you.
Source: BioRUST
Step 1: Open up a stock image in Photoshop of any size and/or format. The best results for this tutorial usually involve large images with a definitive horizon or absolutely straight edge (such as the brickwork of a building, if you wish to correct the skew in an image of a building, etc).
Step 2: Select the Measure Tool from the tools palette. If you are having trouble finding it, its underneath the EyeDropper Tool.
Step 3: To perform the rotation, simply select Image > Rotate Canvas > Arbitrary from the main menu. The Measure Tool will have already entered in the correct angle of rotation for us. Just make sure that the direction of rotation is set correctly (i.e. either clockwise or counter-clockwise), and press OK to rotate your image.
Step 4: Now that you have the results of your rotation, complete with corrected horizon, all you need to do is knock out the black areas with the Crop Tool, a skilfully-used border, or even the Clone Stamp Tool. Have fun!
