Click on the right-pointing arrow you will see in the top right of the Gradient Editor. Select Special Effects from the list that will show up on screen. This option is near the bottom of the list. Photoshop will ask you if you want to replace the gradients initially found in the list with the Special Effects or you simply want to append them to the initial list. In order to have both the initial gradients, and the new ones, the special effects, select “Append”.

You will find the gradient you will use (named “Russell’s Rainbow”) at the last position after you have added the Special Effect gradients. Select Russell’s Gradient by clicking on it.

Because you want your rainbow to also have the arced shape of a rainbow and not only its appearance, you must choose the Radial Gradient option found in the Options Bar at the top of the screen.

The next step is to drag out a rainbow with the help of the Gradient tool. In order to do this, you must have the “Rainbow” layer selected in the Layers palette, the “Russell’s Rainbow” gradient loaded and the Gradient tool in hand. Simply drag out a gradient to create your initial rainbow. You will want your rainbow’s lower end to appear as touching the earth in your image. Position the mouse almost to the end of the picture (right or left, depending to where you want to start your rainbow) near the down bottom. After setting the initial point, click and drag the mouse to a point on the picture where you want to position your rainbow’s lower end. You want to constrain the gradient horizontally. To accomplish this,, hold your Shift key down as you drag.

Do not worry if your result until now does not look like any rainbow you have seen in nature, as you are not done yet. he next step is to change the “Rainbow” layer Blend Mode. Be sure you the “Rainbow” layer is still selected. Go to the Layer Blend Mode options. You will find the setting in the top left of the layers palette. Click on the down pointing arrow next to the “Normal” word. Select the “Screen” option from the list that will appear on the screen.