To draw our raimbow the first thing we need is a place to put it. Find a picture and open it.

Lets select the Gradient Tool and then we will click on the circled icon to find the right gradient.

When the new window pops up, click on the circled icon and a menu will open up. Select special effects from it.

When you see a pop up like the one on the picture, click OK.

Now we need to select the Gradient know as Russell's Rainbow. Create a new layer Layer-New-Layer and we are ready to draw.

With that gradient selected, we now pick the gradient type. We need a new layer to put the rinbow in so we go to Layer-New-Layer and we draw a line from left to right and release the mouse.

You have now drawn your rainbow. You need to change the layer mode to Screen.

We apply a little Gaussian Blur clicking on Filter-Blur-Gaussian Blur and find the right level for your picture.

We will apply another gradient to the rainbow layer by following the numbers: Select the colors, select the Gradient Tool, Find the gradient, select the second icon, and choose the gradient type.

Now click on Layer-Add Layer Mask-Revela All and draw a line from top to bottom. If you do not like what you get, click Edit-Undo and do it again until you get what you want.

And here is the final result.