Photoshop CS 3 has added two new Blend Modes -- Darker Color and Lighter Color, which are not found in the earlier versions. These two Blend Modes can be very useful in blending two images and also for a trick or two. Let me demonstrate.
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Step 1 : I have opened this image in Photoshop CS 3.
Step 2 : Click the New Fill or Adjustment Layer button (red arrow). Hover your cursor over the button and you can read its name.
Step 3 : This popup appears. From it click Black and White. 
Step 4 : The Black and White dialogue box opens.
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Step 5 : I wish to retain the Blue in the image. I have increased the Blue to 88 % as well as the Cyan to 92 %. Then I have decreased the red to to -48 and clicked ok.
The image changes to Black and White.
Step 6 : Click where the arrow points to in the Layers pallette to reveal the Blend Modes. I chose Darker Color.
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Step 7 : Now only the Blues remain.
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Now instead of the settings in Step 7, I have increased the Reds to 50 and reduced the Yellows, Greens, Cyans, Blues and Magentas to -60. I am clicking ok. I am then changing the Blend mode to Darker Color.
Now only the red is visible.
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For another experiment I am changing the Reds to 50 and Cyans and Blues to 60. I am changing the Yellows, Greens and Magentas to -60, then clicking ok. I am then changing the Blend Mode to Darker Color.
Now only the reds and Blue are visible.
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Now I have opened these two photos which I took with my mobile camera.
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I am dragging the image of the foliage to the photo of the building.
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The foliage photo now covers the photo of the building.
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I am changing the Blend Mode to Lighter Color.
And the foliage looks like a tree growing outside the building.
Click the New Style or Fill Layer Button shown in Step 2 and click on Levels.
The Levels dialogue box opens. Click on the Black slider on the left and drag it to just where the 'mountain' begins.
I have dragged the Black slider to where the 'mountain' begins.
Look how vivid the image has become.
I have now clicked on the New Fill or Adjustment Layer Button and clicked on Curves.
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The Curves dialogue box opens. Click on the preset i.e. where the red arrow points to reveal a dropdown box.
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From the drop down I have clicked on Increase Contrast. Then clicked ok.
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The contrast has increased. This is the fun of CS 3 -- great effect with little effort. That is all. Try it out.