One of the most interesting things about Photoshop is that there are often many different ways to accomplish the same task. Take for example changing the colour of the petals in the flower photograph below. You could use the Magic Wand to make a selection and then use curves to colour correct, or you could draw a path around the petals with the Pen tool and use Colour Balance, or even just use Hue and Saturation to shift the colour without making a selection at all. With the emphasis these days on working with Layers it's very easy to forget about the Channels Palette. By looking at each individual Channel of an image you might find the basis of a selection right under your nose. This technique is of course image dependant, in that is it relies on the relationships of colour throughout the image. You'll find some images will work much better than others. The idea is to find one Channel that we can use as a basis to create a selection from.

To begin, simply go to your Channels Palette and click on each Channel name one by one to examine the contrasts they contain. Starting with the Red Channel below...

Next is the Green Channel. Not too much exciting going on contrast wise with the Red or Green Channels.

Now finally with the Blue Channel we can see there is quite alot of contrast compared to the Red and Green Channels. Since the green og the laeves contain yellow and the petals alot of blue, yellow areas will be dark and blue areas light. By duplicating this Channel we can use it as a basis to create our Alpha Channel for our Selecion.

To duplicate the Blue Channel simply drag the Channel onto the new Channel icon in the Channel Palette. By default it is simply named Blue Copy, you can easily rename it if you like. Then go to Image-Adjust-Threshold.

The Threshold command will turn the detailed grayscale information contained in the Alpha Channel into pure Black and White pixels. By adjusting the Threshold Level slider you can visually see what pixel information is kept or lost. You usually can't make a perfect Channel containing just your selection first time out. Some cleanup with the Selection and Painting tools will be required.

Now after cleanup is complete it is always a good idea to soften the edges of the Alpa Channel. If you don't soften the edges any enhancments to the final image could look too harsh and cookie cutter like. How much softening is of course resulution dependant. Low resolution images can get away with using just the Blur filter, higher resolution images will most likely require the use of the Gausian Blur filter which will give much more control of how much softening will occur.

Now with the Alpha Channel completed it is just a matter of loading it as a Selection on the image. A quick way to load a Selection from an Alpha Channel is to click on the composite RGB Channel in the Channel Palette to select it, then Command/Control click the Alpha Channel to load it as a selection.

Although this technique may require a little extra work, it can be well worth the extra bit of effort. Having an alpha channel containing a selection can be quite usefull in many situations such as ion Save for Web, and in the Extract dialogue, not to mention use in Video applications. The final image below.

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