I have just started writing second section of articles on Photoshop. The articles are still of the beginner level but soon I will also start covering more advanced once. Just give me some time 
You can check my first semi advanced photoshop tutorial on How to create a shadow of any object in an image.
In this Adobe® Photoshop tutorial, we will see how to create a realistic rainbow to your natural picture. We will take a sample image as shown below for demo purpose. You are free to choose any other picture of your own, which suits the theme. Take a sample as shown below:
Now you are going to add a new layer to the thumbnail. This can be done using your "Layers Palette" , by clicking on the "Create a new layer". This is found at the bottom of the "Layers Palette". Name the layer as your choice. Now select the "Gradient" tool (Check "Linear Gradient") from the toolbar. You will be seeing a dialog box being opened like this.
Pick the color from the Drop-down box, and select the color palette, where you can see the 256 colors over headed. Now choose the rainbow style, which contains seven colors. Then click "Ok" to apply the gradient over your picture. By holding you shift key, drag the gradient such that the Gradient effect gets filled up in the middle as shown.
Now move to the "Polar Coordinates" from Filter --> Distort --> Polar Coordinates. Select now the "Rectangular to Polar" option and then press "Ok" . The gradient you have created will be now transformed to an elliptical/circle shapes (Which depend on the shape of your thumbnail chosen) .Now Zoom Out to the appropriate Percentage (%) to view the full image with the gradient.
Now activate "Free Transformation" tool by pressing Ctrl + T. And move the gradient to the bottom right side of your picture. Now the Rainbow is created. But what we need is to make it still real. So blur your gradient still more. You may use either "Blur" or "Gaussian Blur" from Filter-->Blur-->Gaussian blur . You may even adjust the "Opacity" of your layer from the "Layers Palette".
To apply still more effects, You can use an ordinary "Brush" tool from the tool box and then brush over the gradient, With the settings of "Opacity" 100% and the size of brush is less than the size of the gradient. The final image with rainbow is created.
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