Artificial Rainbows Tutorial Author - Man1c M0g (http://forums.biorust.com/member.php?userid=1)

Rainbows are beautiful and majestic, but have a reputation for being almost impossible to plan into any photo shoot.  Unlike other scene elements which can be manipulated at will, rainbows are natural phenomenon and do not obey the machinations of mankind.  But why spend hours fighting with nature, when you can simply take a good photograph and add a rainbow electronically at a later date?
 

Step 1: Open up a stock image in Photoshop of any size and/or format. Please note that the image to my right was grabbed from the excellent SXC.HU archive - where there excellent pictures aplenty! NB.  Remember that rainbows are a trick of the light produced when sunlight diffracts through fine droplets of water.  With this in mind, make sure that your source image has these elements in abundance, or else your rainbow will look blatantly artificial!

Step 2: Create a new layer above the Background and call it Rainbow.  Select the gradient tool and then click the gradient itself in the options bar to choose your gradient.  Thankfully, Photoshop comes with an excellent rainbow-style gradient, but it is not loaded as default.  So, click on the little arrow in the top-right corner of the gradient picker, choose 'Load Gradients'  and then select the 'Special Effects' set in your PS folder.  This set will now load and a gradient named 'Russell's Rainbow' will appear (as shown opposite).

Step 3: Select Russell's Rainbow gradient, ensuring that your gradient tool is set to a radial form, and the mode is Normal.  With the Rainbow layer active in the layers palette, click and drag across your image to create your rainbow.   If you are not happy with the size and/or position, just click + drag again until you have the desired placement. Now set your layer blending mode to Screen.  You should end up with an image that resembles mine on the left.

Step 4: To improve realism and finish off the rainbow, we need to mimic nature's own complex fading effect.  To accomplish this, make sure the Rainbow layer is active in the layers palette and click on the 'Add Layer Mask' button () at the bottom of the layers palette. With the layer mask active (the white square to the right of the Rainbow layer thumbnail in the layers palette), select a black-to-white linear gradient and drag a line as shown in the example opposite.

Step 5: It can take a while to get the fade positioning correct, so feel free to keep trying until you have the desired effect.  Each drag overrides the previous one, so its easier to be as pedantic as you like about your final effect. And that's it!  Do remember, though, what I said at the start of this tutorial regarding the appropriate choice of source image.  After all, nothing looks quite as daft as a rainbow in the desert!  Bear realism in mind, however, and the sky is your limit.  Have fun!